EXPERIMENT AND NUMERICAL SIMULATION OF CO2 CONVECTIVE HEAT TRANSFER COEFFICIENT IN MICROCHANNEL EVAPORATOR

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HCMC UNIVERSITY OF TECHNOLOGY AND EDUCATION
FACULTY OF VEHICLE AND ENERGY ENGINEERING

UNDERGRADUATE THESIS

EXPERIMENT AND NUMERICAL SIMULATION OF CO2
CONVECTIVE HEAT TRANSFER COEFFICIENT IN
MICROCHANNEL EVAPORATOR

AUTHORS:
BATUNG PHAM 15147142
BAPHUOC LE

15147118

ADVISOR:
ASSOC. PROF. DR. THANHTRUNG DANG

HCMC, July 2019

HCMC UNIVERSITY OF TECHNOLOGY AND EDUCATION
FACULTY OF VEHICLE AND ENERGY ENGINEERING
DEPARTMENT OF THERMAL ENGINEERING

UNDERGRADUATE THESIS

Topic

EXPERIMENT AND NUMERICAL SIMULATION OF CO2
CONVECTIVE HEAT TRANSFER COEFFICIENT IN

MICROCHANNEL EVAPORATOR

AUTHORS:
T. B. PHAM15147142
P. B. LE

15147118
ADVISOR:

ASSOC. PROF. DR. THANHTRUNG DANG

HCMC, July 2019

TRƯỜNG ĐH SƯ PHẠM KỸ THUẬT

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

TP. HỒ CHÍ MINH

Độc Lập – Tự Do – Hạnh Phúc

KHOA CƠ KHÍ ĐỘNG LỰC
TP. Hồ Chí Minh, ngày......tháng......năm......

NHIỆM VỤ ĐỒ ÁN TỐT NGHIỆP
Họ tên sinh viên:

1. Lê Bá Phước

MSSV: 15147118

(Email:
2. Phạm Bá Tùng

Điện thoại: 01676985115)

MSSV: 15147142

(Email:

Điện thoại: 0913536826)

Ngành: Cơng nghệ Kỹ thuật Nhiệt
Khóa: 2015

Lớp: 159470A

1. Tên đề tài.
Nghiên cứu thực nghiệm và mô phỏng số hệ số truyền nhiệt đối lưu của CO 2 trong thiết
bị bay hơi của hệ thống điều hịa khơng khí kênh micro.
2. Nhiệm vụ đề tài.
+ Nhiệm vụ 1: Tìm hiểu về công nghệ kênh micro.
+ Nhiệm vụ 2: Nghiên cứu và mô phỏng số ảnh hưởng của hệ số truyền nhiệt đối lưu
của CO2 với hiệu suất làm việc của hệ thống.
3. Sản phẩm của đề tài.
+ Quyển báo cáo đồ án.
+ Bài mô phỏng số.
4. Ngày giao nhiệm vụ đề tài: 18/2/2019.
5. Ngày hoàn thành nhiệm vụ: 23/7/2019.

TRƯỞNG BỘ MÔN

CÁN BỘ HƯỚNG DẪN

1

TRƯỜNG ĐH SƯ PHẠM KỸ THUẬT

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

TP. HỒ CHÍ MINH

Độc Lập – Tự Do – Hạnh Phúc

KHOA CƠ KHÍ ĐỘNG LỰC
Bộ mơn.........................

PHIẾU NHẬN XÉT ĐỒ ÁN TỐT NGHIỆP
(Dành cho giảng viên hướng dẫn)
Họ và tên sinh viên:..................................................MSSV:
....................................................................................
Họ và tên sinh viên:..................................................MSSV:
....................................................................................
Tên
đề
tài:
.............................................................................................................................................
...........................................................................................................................................
Ngành

đào
tạo:
.............................................................................................................................................
Họ
và
tên
GV
hướng
dẫn:
.............................................................................................................................................
Ý KIẾN NHẬN XÉT
1. Nhận xét về tinh thần, thái độ làm việc của sinh viên (không đánh máy).
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
2. Nhận xét về kết quả thực hiện của ĐATN (không đánh máy).
2.1. Kết cấu, cách thức trình bày ĐATN.
...........................................................................................................................................
...........................................................................................................................................
2

...........................................................................................................................................
...........................................................................................................................................
2.2. Nội dung đồ án.
(Cơ sở lý luận, tính thực tiễn và khả năng ứng dụng của đồ án, các hướng nghiên cứu có thể
tiếp tục phát triển)

được

Hình thức và kết cấu ĐATN

30

Đúng format với đầy đủ cả hình thức và nội dung của

10

các mục

2.

Mục tiêu, nhiệm vụ, tổng quan của đề tài

10

Tính cấp thiết của đề tài

10

Nội dung ĐATN

50

Khả năng ứng dụng kiến thức toán học, khoa học và

5

kỹ thuật, khoa học xã hội…
Khả năng thực hiện/phân tích/tổng hợp/đánh giá

10

Khả năng thiết kế chế tạo một hệ thống, thành phần,

15

hoặc quy trình đáp ứng yêu cầu đưa ra với những ràng
buộc thực tế.
Khả năng cải tiến và phát triển

15

Khả năng sử dụng công cụ kỹ thuật, phần mềm

5

chuyên ngành…
3.

Đánh giá về khả năng ứng dụng của đề tài

10

4.

Sản phẩm cụ thể của ĐATN

10

Tổng điểm

100

4. Kết luận:
 Được phép bảo vệ
 Không được phép bảo vệ

TP.HCM, ngày

tháng

năm 2019

Giảng viên hướng dẫn
(Ký, ghi rõ họ tên)

4

TRƯỜNG ĐH SƯ PHẠM KỸ THUẬT

CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM

TP. HỒ CHÍ MINH

Độc Lập – Tự Do – Hạnh Phúc

KHOA CƠ KHÍ ĐỘNG LỰC
Bộ mơn.........................

PHIẾU NHẬN XÉT ĐỒ ÁN TỐT NGHIỆP
(Dành cho giảng viên phản biện)
Họ và tên sinh viên:..................................................MSSV:
....................................................................................
Họ và tên sinh viên:..................................................MSSV:
....................................................................................
Tên
đề
tài:
.............................................................................................................................................
...........................................................................................................................................
Ngành
đào
tạo:
.............................................................................................................................................
Họ
và
tên
GV
phản
biện:
(Mã
GV)
.............................................................................................................................................
Ý KIẾN NHẬN XÉT
1. Nhận xét về tinh thần, thái độ làm việc của sinh viên (không đánh máy).

...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
2. Nhận xét về kết quả thực hiện của ĐATN (không đánh máy).
2.1. Kết cấu, cách thức trình bày ĐATN.
...........................................................................................................................................
...........................................................................................................................................
5

...........................................................................................................................................
...........................................................................................................................................
2.2. Nội dung đồ án.
(Cơ sở lý luận, tính thực tiễn và khả năng ứng dụng của đồ án, các hướng nghiên cứu có thể
tiếp tục phát triển)

...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
2.3. Kết quả đạt được.
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
...........................................................................................................................................
2.4. Những tồn tại (nếu có).
...........................................................................................................................................
...........................................................................................................................................

2.

Mục tiêu, nhiệm vụ, tổng quan của đề tài

10

Tính cấp thiết của đề tài

10

Nội dung ĐATN

50

Khả năng ứng dụng kiến thức toán học, khoa học và

5

kỹ thuật, khoa học xã hội…
Khả năng thực hiện/phân tích/tổng hợp/đánh giá

10

Khả năng thiết kế chế tạo một hệ thống, thành phần,

15

hoặc quy trình đáp ứng yêu cầu đưa ra với những ràng
buộc thực tế.
Khả năng cải tiến và phát triển

15

Khả năng sử dụng công cụ kỹ thuật, phần mềm

5

chuyên ngành…
3.

Đánh giá về khả năng ứng dụng của đề tài

10

4.

Sản phẩm cụ thể của ĐATN

10

Tổng điểm

100

4. Kết luận:
 Được phép bảo vệ
 Không được phép bảo vệ

TP.HCM, ngày

tháng năm 2019

Giảng viên phản biện
(Ký, ghi rõ họ tên)

7

Acknowledgements
We’re thankful to Assoc. Prof. Dr. Thanhtrung Dang for preparing micro-scale
evaporator for our research and being our advisor during the experiment as well as in our
need of proofreading and correcting the thesis; Kiencuong Giang for help with
proofreading thesis format; M. Sc. Tronghieu Nguyen and M. Sc. Hoangtuan Nguyen for
help with numerical simulation.

8

Abstract
The term microchannel came from micro-technology developed in electrical and
electronic engineering. The very first conceptual idea of micro-scale technology was
developed by D. B. Tuckerman et. al. after they had scaled down the dimensions of a
conventional plate-fin liquid-cooled heat sink to micrometers. Other studies have been
conducted to estimate pressure loss due to microchannel structure in regard to improve
micro refrigeration system performance. However, most of mentioned studies have
shown poor results in prediction possibility.
Since the day the scientists were aware of pessimistic global warming rate caused by
CFCs, HFCs and HCFCs, hundreds of researches on alternative refrigerants have
implemented. R744 (CO2) is one of top tier alternative refrigerants with proven high
performance and low global warming potential. This study will concentrate on

investigating R744 heat transfer coefficient and overall performance of R744 air
conditioning system both via experimentation and numerical simulation.

9

Content

Acknowledgements...................................................................................................viii
Abstract....................................................................................................................... ix
Content......................................................................................................................... x
Nomenclature............................................................................................................xiii
List of figures.............................................................................................................. xv
List of tables.............................................................................................................xvii
Chapter 1. OVERVIEW..............................................................................................1
1.1. Review of related literature..................................................................................1
1.2. Research background.........................................................................................11
1.3. Authors’ achievement.........................................................................................11
1.4. Methodology......................................................................................................12
1.5. Research objective.............................................................................................12
Chapter 2. INTRODUCTION...................................................................................13
2.1. Microchannel and micro heat exchanger............................................................13
2.1.1. A brief history of micro technology...............................................................13
2.1.2. Micro heat exchangers classification...........................................................14
2.1.3. Micro HX’s advantages and disadvantages compared to those of
conventional HX.....................................................................................................14
2.2. Thermodynamic properties of R744...................................................................15
2.3. Heat transfer and heat transfer coefficient fundamentals....................................16
2.4. Air conditioning fundamentals...........................................................................17
2.4.1. Sensible heat.................................................................................................17

2.4.2. Latent heat....................................................................................................17
2.5. Numerical simulation with COMSOL Multiphysics..........................................18
2.5.1. L-VEL and yPlus...........................................................................................18
10

2.5.2. Spalart-Allmaras..........................................................................................18
2.5.3. K-ε................................................................................................................18
2.5.4. K-ω............................................................................................................... 18
2.5.5. Low Reynolds Number k-ε............................................................................18
2.5.6. Shear stress transport (SST).........................................................................18
Chapter 3. DESIGN AND METHODOLOGY........................................................19
3.1. Experimental system..........................................................................................19
3.1.1. Semi-hermetic single-stage reciprocating compressor.................................20
3.1.2. Evaporator...................................................................................................21
3.1.3. Gas cooler....................................................................................................22
3.1.4. Throttling valve............................................................................................23
3.1.5. Electric control box......................................................................................23
3.2. Measurement instruments..................................................................................24
3.2.1. Pressure gauges............................................................................................24
3.2.2. Temperature and humidity indicators...........................................................24
3.2.3. Anemometer and clamp meter......................................................................25
3.3. Experimental data collection process.................................................................25
3.3.1. Operating the system....................................................................................25
3.3.2. Completing the operation.............................................................................27
3.4. Numerical simulation process............................................................................27
3.4.1. Model environment setup..............................................................................27
3.4.2. Geometrical object creation.........................................................................28
3.4.3. Physics boundary conditions definition........................................................29
Chapter 4. RESULTS AND DISCUSSION..............................................................31

4.1. Measurement data..............................................................................................31
4.2. CHTC’s affection on system performance investigation methodology..............34
4.2.1. Referenced constants....................................................................................34
4.2.2. R744 CHTC () calculation methodology......................................................34
11

4.2.3. COP calculation...........................................................................................37
4.2.4. Other related quantities investigation...........................................................37
4.3. Experimental results analysis.............................................................................38
4.4. Numerical simulation results analysis................................................................42
4.4.1. In terms of temperature................................................................................42
4.4.2. In terms of pressure......................................................................................43
4.4.3. In terms of phase change..............................................................................44
Chapter 5. CONCLUSION AND RECOMMENDATION.....................................45
5.1. Conclusion.........................................................................................................45
5.2. Recommendation...............................................................................................45
References................................................................................................................... 46
Appendix....................................................................................................................50

12

Nomenclature

A. Abbreviation
+ COP: coefficient of performance.
+ CHTC: convective heat transfer coefficient.
+ DTTS: digital thermometer thermocouple sensor.
+ GWP: global warming potential.

+ HX: heat exchanger.
+ LMTD: logarithmic mean temperature difference.
+ SST: shear stress transport

B. Latin letter
+ cp: isobaric specific heat [J/kg.oK]
+ F: heat transfer area/the surface in which heat transfer takes place (m2)
+ G: mass flow rate (kg/s)
+ h: enthalpy (kJ/kg)
+ hfg: latent heat of vaporization (kJ/kg)
+ I: electric current (A)
+ k: overall heat transfer conductivity (W/m2.oK)
+ N: compressor power consumption (kW)
+ p: pressure (bar)
+ q: heat flux (W/m2)
+ Q: heat load (kW; W)
+ s: entropy (kJ/kg.K)

13

+ t: temperature (oC)
+ U: voltage (V)
+ v: velocity (m/s)
+ v: specific volume (m3/kg)
+ W: absolute humidity (kg/kgd. air)

C. Greek letter
+ α: heat transfer coefficient (w/m2.oK)
+ ε: adjustment factor

+ δ: thickness (m; mm)
+ ∆: algebraic difference
+ λ: thermal conductivity (W/m.oK)
+ ρ: density (kg/m3)
+ φ: relative humidity (%)

14

List of figures

Figure 2.1. Mollier diagram of R744..........................................................................15
Figure 3.1. Schematic diagram of the experimental system........................................19
Figure 3.2. Dorin compressor......................................................................................20
Figure 3.3. Compressor’s actual size...........................................................................21
Figure 3.4. Experimental evaporator...........................................................................22
Figure 3.5. Evaporator’s actual size [21].....................................................................22
Figure 3.6. The front (left) and back (right) of the experimental gas cooler...............22
Figure 3.7. Experimental throttling valve....................................................................23
Figure 3.8. Electric control box...................................................................................23
Figure 3.9. p1, p2 (left) and P3, p4 (right) pressure gauges............................................24
Figure 3.10. (From left) Thermo hygrometer, DTTS, digital temperature meter
and temperature humuidity meter................................................................................24
Figure 3.11. Anemometer (left) and clamp meter (right)............................................25
Figure 3.12. Powering the control box (left) and other components (right)................26
Figure 3.13. Switching on evaporator fan (left), gas cooler fan (middle) and
compressor (right)........................................................................................................26
Figure 3.14. Adjusting fan speed (left) and collecting air velocity values
(right)........................................................................................................................... 27
Figure 3.15. Model environment setup.......................................................................28

Figure 3.16. Geometrical object..................................................................................28
Figure 3.17. Geometrical object imported from Autodesk Inventor software.............29
Figure 4.1. Room and evaporator inlet temperature in refer to suction and
discharge pressure........................................................................................................32
Figure 4.2. System cycle graphically illustrated by p-h diagram.................................33
Figure 4.3. Air to CO2 heat transfer pattern.................................................................35
Figure 4.4. CO2-based, air-based CHTC and suction pressure correlation..................40
15

Figure 4.5. Cooling load and suction pressure correlation..........................................41
Figure 4.6. COP, overall cooling load and suction pressure correlation......................42
Figure 4.7. Simulation result in temperature (left) and pressure (right) terms.............43
Figure 4.8. Simulation result in phase change terms (left) and thermographic
image of the evaporator backside (right)......................................................................44

16

List of tables

Table 3.1. Specifications of Dorin compressor
Table 3.2. Legends and symbols in Figure 3.3
Table 3.3. Input parameters for boundary conditions
Table 4.1. Measurement data
Table 4.1. Enthalpy values at designated points
Table 4.2. Calculation database in CO2 terms
Table 4.3. Calculation database in air blow terms
Table 4.4. Other quantities calculation database
Table 4.5. Comparison between experiment and simulation for evaporator

temperatures
Table 4.6. Comparison between experiment and simulation for evaporator
pressures
Table 4.7. Comparison between experiment and simulation for evaporator
phase changes
Table A. R744 - CO2 saturation properties (pressure)
Table B. Superheated and transcritical vapor properties of R744-CO2

17

Chapter 1. OVERVIEW

1.1. Review of related literature
This study has been experimentally supported and theoretically based on following
national and international related literature reviews:
Buytaert et. al. [1] studied on microchannel evaporative CO 2 cooling for the upgrade of
the LHCb vertex detector. Research shows that Evaporative CO 2 cooling in silicon
microchannels would meet the stringent requirements for the upgraded LHCb VELO
detector, regarding thermal efficiency, low temperature (−27 oC), high power density (12.9
W) and low mass. Temperature gradients of less than 4 oC can be achieved in the module.
We are addressing the issue of high pressure resistance and reliability to operate in the
detector vacuum.
Li et. al. [2] performed a study on CO 2 gas removal design for a micro passive direct
methanol fuel cell. Result states that To improve the gas management of DMFC, a novel
configuration with super hydrophobic lateral venting microchannels was fabricated,
which allows CO2 gas directly release from MEA diffusion layer. This configuration can
decrease or even avoid CO2 gas passing through the anode flow field and getting into the
methanol solution reservoir, easing the CO2 gas barrier accumulation. A totally enclosed
DMFC was realized with the lateral venting configuration. Performance at high current

density and low methanol concentration is dramatically improved as a result of the low
anode methanol-transport resistance and the anode concentration polarization.
Meanwhile, a more stable output voltage is also demonstrated by the I-V curve of
constant-current-density discharging test, which is also proved by the photos of less
bubbles producing on the anode. Lateral venting configuration presented in this work
offers a new way to optimize the DMFC, which is really worthy to further study. Further
studies will focus on the different arrangement of lateral venting channels, exploring an
optimal design. In addition, expanding its application to larger flow field is also a subject
to be studied.

1

Roychowdhury et. al. [3] carried out a research on Conjugate heat transfer studies on
steam reforming of ethanol in microchannel systems. Study’s conclusion indicates that
complete ethanol conversion is possible if the flue gas temperature is above a minimum
value (~1400 oC). However, the overall thermal efficiency does not vary much beyond a
certain value of flue gas inlet temperature and hence it is important to identify optimum
heating condition for better energy utilization. Even with 100% ethanol conversion
efficiency, hydrogen yield is affected by methanation and water-gas shift reactions
occurring in the bulk of the reforming fluid, depending on the system temperature. At low
reforming fluid temperatures (~600 K), ethanol decomposition and methanation give rise
to high methane concentration and very low hydrogen yield. As mixture temperature
increases, the concentrations of both carbon monoxide and hydrogen increase
significantly. At very high mixture temperatures, hydrogen yield decreases due to reverse
water-gas shift reaction. Thus, a mixture temperature of about 1000 K appears to be
optimum, for maximum hydrogen yield from the reformer system.
Gaudillere et. al. [4] studied on CO2 hydrogenation on Ru/Ce based catalysts dispersed
on highly ordered microchannelled 3YSZ monoliths fabricated by freeze-casting. Ru/Cebased catalysts have been evaluated under two different configurations for CO 2
methanation reaction at 450oC and 500oC. A systematic comparison of fixed-bed catalysts

and catalyst-loaded freeze-cast monoliths confirmed that Ru/Ce-based catalysts are
effective for this reaction and that the monolith configuration decreases the CO
production and as a consequence improves the CH 4 selectivity. As expected, an increase
of the H2 content in the inlet stream shifts the RWGS reaction to the equilibrium and thus
improves CH4 production. Finally a study of the pressure drop through all the catalysts
did not reveal any significant increase of pressure due to carbon deposition and confirms
the possible implementation of freeze-cast porous samples as catalyst support. All these
observations confirm that a good control of the catalyst composition is crucial and
determines the final yield of the reaction. As a first evaluation of these ceramic monoliths
as catalyst support, we can conclude on an interesting opportunity for the optimization of
catalytic processes. Nevertheless, several improvements can be imagined to boost CH 4
selectivity, decrease the carbon deposition or increase the CO 2 conversion: modification

2

of the pores microstructure and size, better control of the infiltration of catalysts
precursors, optimization of the metal loading etc.
Kim et. al. [5] implemented a research on effect of micro-grooves on the two-phase
pressure drop of CO2 in a minichannel tube. Major findings in this study can be
summarized as follows:
+ Using the hydraulic diameter, the pressure drop of CO 2 in the grooved minichannel
could be successfully estimated with conventional correlations for smooth channels.
+ For a liquid phase, the Blasius correlation estimated the pressure drop of both
channels very precisely. Because of a smaller hydraulic diameter, the pressure drop of the
grooved tube was found to be about 1.3 times greater than the smooth tube.
+ In the two-phase experiments, the pressure drop of the grooved channel was found
to be 1.1-1.45 times greater than that of the smooth channel.
+ The experiments with a constant heat flux confirmed that the pressure drop data in
adiabatic condition could be well applied to predict the actual pressure drop in the

evaporator with changing quality.
+ Most of the two-phase pressure drop models showed the mean absolute error of
17e35% for the smooth channel and 13-32% for the grooved channel. The model of
Mishima and Hibiki (1995) showed the best result.
Liang et. al. [6] carried out an investigation of flow boiling heat transfer characteristics
of CO2 in horizontal mini-tube. In this paper, through experimental method, analyzing the
impact of the heat flux, mass flux, the saturation temperature on the heat transfer
coefficient and dryout characteristics, comparing the experimental results with the
theoretical prediction model, obtain following conclusions:
+ Compared with the conventional refrigerants CO 2 has better thermal properties
which makes it acquire higher heat transfer coefficient in minichannel in low-medium
vapor quality area, with the increase of vapor quality the phenomenon of dryout will
happen thereby result in heat exchange performance fall sharply, makes the average heat
transfer coefficient decrease in the whole heat transfer process.
3

+ During the process of these working conditions, the heat flux has a significant
impact on the heat transfer coefficient, the increase of the heat flux enhances the effect of
nucleate boiling heat transfer so that the heat transfer coefficient increases.
+ During the process of these working conditions, the increase of heat flux speeds up
the process of dryout and has impact on the vapor quality of dryout at both starting and
ending stage; with the increase of the mass flux, the starting vapor quality of dryout
declines; the higher the saturation temperature is, the easier the dryout phenomenon
occurs and the starting vapor quality of dryout is lower.
+ Comparing the existing theory with experimental results, its indicates that the
Cheng's model has a higher prediction accuracy and can predict the heat transfer
coefficient well before the phenomenon of dryout occur, the prediction deviation within
±30% and the accuracy is 77.1%.
Jiang et. al. [7] investigated enhancement of CO 2 dissolution and sweep efficiency in

saline aquifer by micro bubble CO2 injection. In this study, supercritical CO2 injection
into a Berea sandstone sample at 0.05 mL/min under reservoir conditions was
investigated using X-ray CT. Experiments with MB and NB CO 2 injection were
conducted. Little MB CO2 was observed near the inlet, suggesting that MBs accelerated
dissolution in the early stage of injection and inlet zone. Smaller saturation levels and
larger volume differences were observed for MB CO 2 compared to NB CO2 in the early
stage because of dissolution. The enhanced dissolution efficiency of MB CO 2 reached
18.7% at 0.02 PV. Moreover, pore space utilization was improved due to higher
permeability of MB CO2 into regions of low porosity and low flow ability. Notably, the
improvement in the utilization efficiency reached 4.9% in low-porosity regions. The
results of this study suggest that MB CO2 sequestration is an efficient method of storing
CO2 in low-porosity regions by enhancing dissolution and improving pore space
utilization.
Liang et. al. [8] studied on enhance performance of micro direct methanol fuel cell by
in situ CO2 removal using novel anode flow field with superhydrophobic degassing
channels. Power densities of the N-Serpentine and N-Spiral m DMFCs were increased by
4

more than 30% and 90% compared to those of the C-Serpentine and C-Spiral m DMFCs,
respectively. This performance increase can be attributed to the more efficient removal of
CO2 bubbles from the AFFs with superhydrophobic degassing channels, which had been
validated by the pressure drop, visual investigation and EIS results. This means that the
new AFF structure developed in this work can remove most of the gaseous CO 2 in situ
before it is released to the flow channels of the fuel, so as to prevent the CO 2 blocking
prior to its occurrence, and finally effectively improve the performance of the DMFCs.
Cheng et. al. [9] performed an analysis of supercritical CO 2 cooling in macro- and
microchannels. Based on the present study, the following conclusions have been reached:
+ Although there are a number of heat transfer correlations for cooling of supercritical
CO2, it is not possible at this point to provide a documented recommendation of which

one(s) is (are) best since few data are presented in a usable format for such comparisons.
It is recommended that further experiments be done over a wide range of test parameters
and proper measurement and data reduction methods be used.
+ Several studies have shown that the Blasius correlation works well for the frictional
pressure drop of supercritical CO2 in both macro- and micro-scale channels. More careful
experimental friction pressure drop data are still needed to further validate this
conclusion because some experimental data are much different from others.
+ Lubricating oil has a very adverse effect on heat transfer and pressure drops.
Generally heat transfer coefficients decrease and pressure drops increase with increasing
oil concentration, by as much as 50% or more. So far, apparently there are no heat
transfer or pressure drop correlations accounting for the effect of oil. In fact, there are
only few studies of the oil effect at supercritical cooling conditions. More experimental
studies are necessary to develop heat transfer and pressure drop database and
correlations.
Reddy et. al. [10] performed numerical Investigation of Micro-channel Based Active
Module Cooling for Solar CPV System. Investigation of channel cooling technologies
employing different flow arrangement, for CPV cooling has been carried out. The micro
channels were found to better at cooling the module and pressure drop was found to be
5

low in straight flow channels, both these facts were combined to develop a combinatory
model with microchannel array removing heat and straight flow channels providing the
flow direction. It was found that microchannels with higher aspect ratios, lower widths
and higher Re result in a low temperature rise of bottom surface and high temperature
uniformity index. As flow progresses in the straight channel containing microchannels its
velocity increases which in turn result in better heat transfer in later microchannels with
similar amount of temperature rise and temperature uniformity even with higher inlet
temperature. The optimized geometry of microchannel for the CPV receiver was found to
be W = 0.5 mm, Length = 12mm and Pitch = 0.5mm. The final results predicts over less

than 10 K rise in temperature of CPV module of dimensions 240 x 240 mm 2, with a
pressure drop of 8.8 kPa along a single channel with six such channels in each modules at
a flow rate of 0.105 lit/s. The overall load was calculated to be 4 W which is
approximately 0.2% of the power produced by the CPV module.
Ducoulombier et. al. [11]’s study of charge reduction experimental investigation of CO 2
single-phase flow in a horizontal microchannel with constant heat flux conditions shows
that for single-phase flow experimentation, the mass velocity of liquid CO 2 is
investigated from 200 kg/m2/s to 1400 kg/m2/s. The experimental data are pretty well
fitted by the Churchill’s law considering the roughness of 0.8 mm. In laminar flow (Re <
2000) and the turbulent flow (Re > 3500), the measurements correspond to the theoretical
prediction, however the transition regime (2000 < Re < 3000) is exhibiting a larger
discrepancy in the experimental data. In the laminar regime, the Hausen’s correction is
used as indicated previously and the data are quiet well predicted by the theoretical
relation. In turbulent regime, the temperature difference between the CO 2 and the wall
reaches up to 2oC in the best conditions and 0.5 oC in the worst ones. The discrepancy
observed for some experimental points are correlated to the lowest temperature
differences where the estimation of the experimental heat transfer coefficient is sensitive
to the accuracy in determining of the CO2 bulk temperature and thus to the heat losses.
The experiment of Han et. al. [12] was implemented on development of microchannel
heat exchanger applied in air-conditioning system, which stated its practical result: the
traditional heat transfer performance can be improved by using the microchannel
6

EXPERIMENT AND NUMERICAL SIMULATION OF CO2 CONVECTIVE HEAT TRANSFER COEFFICIENT IN MICROCHANNEL EVAPORATOR

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