Boiling in Microchannels with Refrigerant Mixtures

2010 ◽  
Vol 34-35 ◽  
pp. 576-581
Author(s):  
Zi Cheng Hu ◽  
Hu Gen Ma ◽  
Xin Nan Song ◽  
Qian Wang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient ◽  
Refrigerant Mixtures

Experiments were conducted to investigate the saturated flow boiling heat transfer characteristics in single micro tube using environmentally acceptable refrigerant mixtures R32 and R134a. Local heat transfer coefficient was measured and boiling heat transfer mechanisms were discussed for a range of heat flux (3-65 kW/m2) , mass flux (860-4816 kg/m2•s) and quality (0-0.9). These characteristics indicated that the local heat transfer coefficient was greatly dependent of heat flux and independent of mass flux and quality in the nucleate boiling regime, which was oppsite to that in forced convection regime, and deterioration of boiling heat transfer occurred in the local dry-out regime. In addition, a correlation for nucleate dominant boiling in micro tube was developed ,which included the effects of heat flux and fluid property and showed some success with the data of this study within a 20% random error band.

Local Heat Transfer Coefficient During Condensation in a 0.8 mm Diameter Pipe

2006 ◽  
Author(s):  
Alberto Cavallini ◽  
Davide Del Col ◽  
Marko Matkovic ◽  
Luisa Rossetto
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Operating Conditions ◽  
Local Heat Transfer Coefficient ◽  
Wide Range ◽  
The Heat Transfer Coefficient

The first preliminary tests carried on a new experimental rig for measurement of the local heat transfer coefficient inside a circular 0.8 mm diameter minichannel are presented in this paper. The heat transfer coefficient is measured during condensation of R134a and is obtained from the measurement of the heat flux and the direct gauge of the saturation and wall temperatures. The heat flux is derived from the water temperature profile along the channel, in order to get local values for the heat transfer coefficient. The test section has been designed so as to reduce thermal disturbances and experimental uncertainty. A brief insight into the design and the construction of the test rig is reported in the paper. The apparatus has been designed for experimental tests both in condensation and vaporization, in a wide range of operating conditions and for a wide selection of refrigerants.

Boiling Heat Transfer of R-134a Flowing in Horizontal Small-Diameter Tubes

2007 ◽  
Author(s):  
Shizuo Saitoh ◽  
Hirofumi Daiguji ◽  
Eiji Hihara
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Small Diameter ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Tube Diameter ◽  
Local Heat Transfer Coefficient ◽  
R 134A

The boiling heat transfer of refrigerant R-134a flow in horizontal small-diameter tubes with inner diameter of 0.51, 1.12, 2.0 and 3.1 mm was experimentally investigated. Local heat transfer coefficient and pressure drop were measured at a heat flux ranging from 5 to 39 kW/m2, mass flux from 100 to 450 kg/m2s, inlet vapor quality from 0 to 0.2, and evaporating pressure of 0.49 MPa, 3.0 and 3.7 MPa. Results showed that the local heat transfer coefficient tends to decrease at lower vapor quality with the decrease in tube diameter. The effect of heat flux on local heat transfer coefficient becomes significant with the decrease in tube diameter, while the effect of mass flux is weak especially for small diameter tube. With decreasing tube diameter, the flow inside it approached homogeneous flow, and the contribution of forced convective evaporation to the boiling heat transfer decreases. With the increase in pressure near the critical pressure (3.0 to 3.7 MPa), the heat transfer coefficient increased, and the effect of mass flux on the heat transfer coefficient became weak. These results implied that the nucleate boiling was dominant under high pressure conditions. A modified Chen-type correlation taking into account the effect of tube diameter was proposed for the prediction of boiling heat transfer of R-134a in horizontal tube. The effect of tube diameter on flow boiling heat transfer coefficient was characterized by the Weber number in gas phase. Comparison with experimental results showed that this correlation could be applied to a wide range of tube diameters (0.5 to 11 mm) and pressure conditions (reduced pressure from 0.1 to 0.9).

Modeling Bubble Growth in Micro-Channel Cooling Systems

2007 ◽  
Author(s):  
Joshua L. Nickerson ◽  
Martin Cerza ◽  
Sonia M. F. Garcia
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Heat Conduction Equation ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

The solution of the heat conduction equation in the liquid layer beneath a moving bubble’s base and the resulting local heat transfer coefficient are presented. An analytical model was constructed using separation of variables to solve the heat conduction equation for the thermal profile in the liquid film beneath the base of a bubble moving through a microchannel at a given velocity. Differentiating the resulting liquid thermal profile and applying the standard definition for the local heat transfer coefficient resulted in a solution for local heat transfer coefficient as a function of bubble length. Analysis included varying pertinent parameters such as film thickness beneath the bubble base, wall heat flux, and superheated temperature in the microchannel. Water and FC-72 were analyzed as prospective coolant fluids. Analytical data revealed that as the superheated temperature in the microchannel increases, local heat transfer coefficients increase and arrive at a higher steady-state value. Increasing wall heat flux achieved the same result, while increasing film thickness resulted in lower heat transfer coefficients. The model indicated that water had superior performance as a coolant, provided the dielectric fluid (FC-72) is not mandated.

THEORETICAL EVALUATION OF NEW PHASE DELAY METHODS FOR MEASURING LOCAL HEAT TRANSFER COEFFICIENTS

1999 ◽  
Vol 23 (3-4) ◽  
pp. 361-376 ◽  
Author(s):  
W. Turnbull ◽  
P. Oosthuizen
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient ◽  
Phase Delay ◽  
Driving Frequency ◽  
One Dimensional

A one-dimensional analytical solution has been derived for unsteady heat conduction within a semi infinite body, of high thermal resistance, that is subject to a surface heat flux that varies periodically with time. The heat flux is assumed to be generated within a thin isothermal coating. The model predicts that a phase delay will develop between the heat flux and the coating thermal response. This phase delay is independent upon the material properties of the substrate and coating, on the heat flux driving frequency, and on the local heat transfer, coefficient. With the exception of this last quantity the other parameters are known a priori, hence if the phase delay can be measured experimentally it can then be used to determine the local heat transfer coefficient. Absolute values of the local coating temperature and local heat flux are not required. Hence calibration of the devices for measuring these quantities should not be required. In contrast to the overall surface temperature, it is predicted that the phase delay angle will attain a steady-state value within a few heat flux cycles, thus reducing the time required obtaining a measurement. Furthermore, the one-dimensional mathematical model that has been developed reduces to those used in previous experimentally validated techniques, when appropriate constants in the boundary condition are used.

Flow Boiling Inside a Single Circular Minichannel: Measurement of Local Heat Transfer Coefficient

2007 ◽  
Author(s):  
Alberto Cavallini ◽  
Stefano Bortolin ◽  
Davide Del Col ◽  
Marko Matkovic ◽  
Luisa Rossetto
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient ◽  
Experimental Apparatus ◽  
Boiling Process

This paper describes a new experimental apparatus for the measurement of the local heat transfer coefficient during flow boiling inside a 0.96 mm internal diameter single round cross section minichannel and reports preliminary heat transfer data taken during flow boiling of R134a. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. This paper also presents a methodology to determine the critical conditions during the flow boiling process when no heat flux is imposed.

CFD Simulation of Heat Transfer in SI Engine Combustion Chamber

2003 ◽  
Author(s):  
Manoochehr Rashidi ◽  
Ali Reza Noori
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Combustion Chamber ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient ◽  
Si Engine ◽  
Maximum Heat

The investigation reported in this paper includes the variation of transient and local heat transfer coefficient and heat flux in the combustion chamber of a spark ignition (SI) engine. Heat transfer characteristics are obtained from the Kiva-3v CFD (Computational Fluid Dynamics) code. Instantaneous results including the variations of mean heat transfer coefficient on the piston surface, combustion chamber, and wall of the cylinder are presented. Moreover, variations of the local heat transfer coefficient and heat flux along a centerline on the piston as well as a few locations on the combustion chamber surface are shown. It is illustrated that maximum heat transfer coefficient on the piston and combustion chamber surfaces varies with location and also it is observed that the initial high rate of increase of heat flux at any position is related to the instant of flame arrival at that position. In this work, the major focus is on the determination of the locations where heat flux and heat transfer are maximum.

Local and Average Heat Transfer Characteristics for a Disk Situated Perpendicular to a Uniform Flow

1985 ◽  
Vol 107 (2) ◽  
pp. 321-326 ◽  
Author(s):  
E. M. Sparrow ◽  
G. T. Geiger
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Stagnation Point ◽  
Radial Distance ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient ◽  
Average Heat ◽  
Radial Profiles

Wind tunnel experiments were performed to determine both the average heat transfer coefficient and the radial distribution of the local heat transfer coefficient for a circular disk facing a uniform oncoming flow. The experiments covered the range of Reynolds numbers Re from 5000 to 50,000 and were performed using the naphthalene sublimation technique. To complement the experiments, an analysis incorporating both potential flow theory and boundary layer theory was used to predict the stagnation point heat transfer. The measured average Nusselt numbers definitively resolved a deep disparity between information from the literature and yielded the correlation Nu = 1.05 Pr0.36 Re1/2. The radial distributions of the local heat transfer coefficient were found to be congruent when they were normalized by Re1/2. Furthermore, the radial profiles showed that the local coefficient takes on its minimum value at the stagnation point and increases with increasing radial distance from the center of the disk. At the outer edge of the disk, the coefficient is more than twice as large as that at the stagnation point. The theoretical predictions of the stagnation point heat transfer exceeded the experimental values by about 6 percent. This overprediction is similar to that which occurs for cylinders and spheres in crossflow.

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