9th IEA Heat Pump Conference

1
9th IEA Heat Pump Conference Annex 33 Workshop 21
May, 2008

• Heat transfer characteristics of carbon dioxide
• flowing in a microfin tube
• - Correlations for HTC and PD at supercritical
  pressure -
• Shigeru KOYAMA, Kyushu University, Japan

2
Background

• From the viewpoints of grovel environmental
  problems,
• Regulation of Refrigerants HFCs
• ? Natural refrigerants such as
  CO2, NH3, HCs
• Low GWP and Zero ODP
• In Japan
• Heat pump type hot water heaters using CO2 as
  refrigerant have been already commercialized
  since 2001, and their market has getting larger
  and larger.
• CO2 is also considered as one of alternative
  refrigerants for domestic air-conditioners.

• Important issues for designing better CO2 heat
  pump systems
• to understand and clarify heat transfer and
  pressure drop characteristics of CO2 flowing
  inside microfin tubes
• to make correlations for HT and PD in microfin
  tubes

3
Objectives

• Experimental study on cooling heat transfer of
  CO2 on the supercritical pressure condition
• Investigate the heat transfer and pressure drop
  
• characteristics of CO2 inside horizontal
  micro-fin tubes
• to clarify effects of geometrical parameters on
  HTC and PD
• to confirm their availability for heat exchanger
  tubes
• Develop correlations of HTC and PD for CO2
  taking into
• account geometrical parameters
• to support a design of heat exchanger

4
Experimental apparatus
Test section
Gas cooler
Evaporator
5
Test section
2064
688
688
688
?P
Flow direction of CO2
500
P
K
K
PT
PT
PT
PT
PT
PT
T
T
T
Heat source
V
K K-type thermocouple T T-type
thermocouple PT Resistance thermometer P
Absolute pressure transducer ?P
Differential pressure transducer V
Volumetric flow meter

• Measuring items
• Refrigerant temperature
• Refrigerant pressure
• Cooling water temperature

• Refrigerant flow rate
• Tube wall temperature
• Refrigerant pressure drop
• Cooling water flow rate

6
Geometrical parameters of test tubes
7
Experimental conditions and data reduction

• Experimental condition

• Definition of heat transfer coefficients, a

Subscripts b CO2 bulk temperature water
cooling water wi Inner wall temperature wo
Outer wall tempera
8
Heat transfer coefficients of grooved tube No.1

• aexp values show peak value near the
  pseudocritical temperature
• aexp values of 8 MPa are greater than those of
  10 MPa near the pseudocritical temperature
• Same tendencies of heat transfer characteristics
  for other grooved tubes

9
Heat transfer enhancement factor

• aexp of grooved tubes are greater than those of
  smooth tube
• Grooved tube No.5 show the highest enhancement
  factor.
• Grooved tubes are available for heat transfer
  enhancement tube in gas-coolerl

10
Comparison of HTCs with a correlation of single
phase flow
HTC correlation of Goto et al.
----------- Experimental condition of Goto et al.
-----------

• Test fluid Water
• Reynolds number 104 105
• Geometrical parameters of test tubes

11
Comparison with the correlation of Goto et al.

• Similar trend on the variation of Re number
• Large deviation against predicted values
• The correlation should be take into consideration
  of effects of physical properties

12
Development of correlation for supercritical CO2

• Concept for the development of correlation
• Introduce the same formula as the
  Dittus-Boelter correlation
• Take into account the effect of physical
  properties
• Subscripts
• b bulk temperature
• f film temperature ( Twi Tb ) / 2

13
Relationship between Re f and Nu f /Pr b0.6
Grooved tube No.1
Grooved tube No.2
Grooved tube No.3
Grooved tube No.5
14
Heat transfer correlation
The present correlation with geometrical
parameters of grooves
15
Results of pressure drop
Goto et al. pressure drop measurement of
single phase water flow
inside 14 kinds of microfin tubes
16
Pressure drop correlation
Comparison of measured frictional pressure drop
with calculated values by the correlation
17
Condensation heat transfer of CO2 in grooved tube
18
Void fraction of CO2 evaporating in grooved tube
By means of quick closing valve method
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Appendix
Definition of Heat transfer coefficient, a
Ws Mass flow rate of cooling water cps Heat
capacity of cooling water Qloss Heat loss at
each subsection ?Z Effective heat transfer
length ?T temperature difference of
cooling water between inlet and outlet
of the subsection ?w Thermal conductivity of
the test tube di Average inside diameter
Where,
20
Appendix
Accuracy of measurement devices
21
Appendix
Physical property of CO2 at supercritical
pressure condition
(a) Heat capacity
(b) Density