Hua Sheng Wang

A Theory of Film Condensation in Horizontal Noncircular Section Microchannels

The paper presents a theoretical model to predict film condensation heat transfer from a vapor flowing in horizontal square and equilateral triangular section minichannels or microchannels. The model is based on fundamental analysis which assumes laminar condensate flow on the channel walls and takes account of surface tension, interfacial shear stress, and gravity. Results are given for channel sizes (side of square or triangle) in the range of 0.5–5 mm and for refrigerants R134a, R22, and R410A. The cases considered here are where the channel wall temperature is uniform and the vapor is saturated at the inlet. The general behavior of the condensate flow pattern (spanwise and streamwise profiles of the condensate film), as well as streamwise variation of local mean (over section perimeter) heat-transfer coefficient and vapor mass quality, are qualitatively in accord with expectations on physical grounds. The magnitudes of the calculated heat-transfer coefficients are in general agreement with experimental data for similar, but nonidentical, channel geometry and flow parameters.

Film Condensation in Horizontal Microchannels: Effect of Channel Shape

The paper gives a progress report on a theoretical study of film condensation in microchannels. The model takes account of surface tension, vapor shear stress and gravity. The effect of channel shape is investigated for condensation of R134a in channels with cross sections: square, triangle, inverted triangle, rectangle with longer side vertical and rectangle with longer side horizontal. The case considered here is where the channel wall temperature is uniform and the vapor is saturated at inlet. For a given mass flux, the local condensate film profile around the cross section is calculated together with the mean heat-transfer coefficient at different distances along the channel. Results are presented here for one vapor mass flux, one vapor temperature and one wall temperature.© 2005 ASME

A Theoretical Model of Film Condensation in Square Section Horizontal Microchannels

This paper presents a theoretical model to predict film condensation heat transfer in square section horizontal microchannels. The model is based on fundamental analysis assuming laminar condensate flow on the channel walls taking account of surface tension, vapour shear stress and gravity. Sample numerical results are given for R134a and channel size (side of square) 1.0 mm. The general behaviour of the condensate flow pattern (spanwise and streamwise profiles of the condensate film), as well as streamwise variation in quality and local mean (over section perimeter) heat-transfer coefficient, are qualitatively in accord with expectations on physical grounds.

Condensation of refrigerants in horizontal microfin tubes: comparison of prediction methods for heat transfer

Abstract A comparison was made between the predictions of previously proposed empirical correlations and theoretical model and available experimental data for the heat transfer coefficient during condensation of refrigerants in horizontal microfin tubes. The refrigerants tested were R11, R123, R134a, R22 and R410A. Experimental data for six tubes with the tube inside diameter at fin root of 6.49–8.88 mm, the fin height of 0.16–0.24 mm, fin pitch of 0.34–0.53 mm and helix angle of groove of 12–20° were adopted. The r.m.s. error of the predictions for all tubes and all refrigerants decreased in the order of the correlations proposed by Luu and Bergles [ASHRAE Trans. 86 (1980) 293], Cavallini et al. [Cavallini A, Doretti L, Klammsteiner N, Longo L G, Rossetto L. Condensation of new refrigerants inside smooth and enhanced tubes. In: Proc. 19th Int. Cong. Refrigeration, vol. IV, Hague, The Netherlands, 1995. p. 105–14], Shikazono et al. [Trans. Jap. Sco. Mech. Engrs. 64 (1995) 196], Kedzierski and Goncalves [J. Enhanced Heat Transfer 6 (1999) 16], Yu and Koyama [Yu J, Koyama S. Condensation heat transfer of pure refrigerants in microfin tubes. In: Proc. Int. Refrigeration Conference at Purdue Univ., West Lafayette, USA, 1998. p. 325–30], and the theoretical model proposed by Wang et al. [Int. J. Heat Mass Transfer 45 (2002) 1513].

Modified theoretical models of film condensation in horizontal microfin tubes

The previously proposed theoretical models of film condensation in horizontal microfin tubes have been modified to describe the characteristics of condensing two-phase flow more accurately. The stratified flow regime and the annular flow regime were considered. For the stratified flow regime, the previously proposed theoretical model was modified to take account of the curvature of stratified condensate due to the surface tension force. For the annular flow regime, a more accurate expression for the interfacial shear stress was incorporated. Generally, the modified theoretical models predicted a lower circumferential average heat transfer coefficient than the previously proposed ones. Comparison of the theoretical predictions with available experimental data for six tubes and five refrigerants revealed that a good agreement (r.m.s error of less than 21.1%) was obtained for all cases when the higher of the two theoretical predictions were adopted as the calculated value.

A Theoretical Study of Film Condensation in Horizontal Microfin Tubes

A stratified flow model of film condensation in helically grooved, horizontal microfin tubes has been developed. The height of stratified condensate was estimated by extending the Taitel and Dukler model for a smooth tube to a microfin tube. For the upper part of the tube exposed to the vapor flow, laminar film condensation due to the combined effects of gravity and surface tension forces was assumed. For the lower part of the tube exposed to the stratified condensate flow, the heat transfer coefficient was estimated by an empirical equation for the internally finned tubes developed by Carnavos. The theoretical predictions of the circumferential average heat transfer coefficient by the present model and previously proposed annular flow model were compared with available experimental data for five tubes and five refrigerants. It was shown that the stratified flow model was applicable to wide ranges of mass velocity and quality as long as the vapor to liquid density ratio was larger than 0.05. Comparison was also made with the predictions of previously proposed empirical equations.

Effect of interphase matter transfer on condensation on low-finned tubes––a theoretical investigation

Abstract The paper gives theoretical results for condensation on low-finned tube in which the temperature drop at the liquid–vapour interface due to interphase matter transfer (interface resistance) is included. The condensation coefficient is taken as unity. Results show, for the case of steam, that in the regions of the fin surface where the condensate film is very thin, the local heat flux can be reduced by a factor of around 2 when the interface resistance is included. Theoretical results for the top of the tube show a significant drop in average vapour-to-surface heat-transfer coefficient with decrease in vapour pressure (around half associated with interface resistance and half due to fluid property variation) in line with earlier measurements.

Condensation of refrigerants in horizontal microfin tubes: comparison of correlations for frictional pressure drop

Abstract This paper presents a comprehensive comparison of eight previously proposed correlations with available experimental data for the frictional pressure drop during condensation of refrigerants in helically grooved, horizontal microfin tubes. Calculated values are compared with experimental data for seven refrigerants (R11, R123, R134a, R22, R32, R125 and R410A) and eight tubes and with mass velocity from 78 to 459 kg/m 2 s. The tubes had inside diameter at the fin root between 6.41 and 8.91 mm; the fin height varied between 0.15 and 0.24 mm; the fin pitch varied between 0.34 and 0.53 mm and helix angle between 13 and 20°. The results show that the overall r.m.s. deviations of relative residuals of frictional pressure gradient for all tubes and all refrigerants taking together decreased in the order of the correlations of Nozu et al. [Exp. Therm. Fluid Sci. 18 (1998) 82], Newell and Shah [Refrigerant heat transfer, pressure drop, and void fraction effects in microfin tubes. In: Proc. 2nd Int. Symp. on Two-Phase Flow and Experimentation, vol. 3. Italy: Edizioni ETS; 1999. p. 1623–39], Kedzierski and Goncalves [J. Enhanced Heat Transfer 6 (1999) 161], Cavallini et al. [Heat Technol. 15 (1997) 3], Goto et al. (b) [Int. J. Refrigeration 24 (2001) 628], Choi et al. [Generalized pressure drop correlation for evaporation and condensation in smooth and microfin tubes. In: Proc. of IIF-IIR Commision B1, Paderborn, Germany, B4, 2001. p. 9–16], Haraguchi et al. [Condensation heat transfer of refrigerants HCFC134a, HCFC123 and HCFC22 in a horizontal smooth tube and a horizontal microfin tube. In: Proc. 30th National Symp. of Japan, Yokohama, 1993. p. 343–5], and Goto et al. (a) [Int. J. Refrigeration 24 (2001) 628], i.e., this final correlation (Goto et al. (a)) gives the best overall representation of the data.

Film Condensation in Horizontal Triangular Section Microchannels: A Theoretical Model

The paper presents a theoretical model to predict film condensation heat transfer from a vapor flowing in a horizontal tube with equilateral triangular section minichannels or microchannels. The model is based on fundamental analysis which assumes laminar condensate flow on the channel walls and takes account of surface tension, vapor shear stress and gravity. The case considered here is where the channel wall temperature is uniform and the vapor is saturated at inlet. Sample numerical results are given for the channel size (side of triangle) of 1.0 mm and for refrigerant R134a. The general behaviour of the condensate flow pattern (spanwise and streamwise profiles of the condensate film), as well as streamwise variation in quality and local mean (over section perimeter) heat-transfer coefficient, are qualitatively in accord with expectations on physical grounds.Copyright

Successive inverse and normal magnetocaloric effects in HoFeSi compound

Magnetic properties and magnetocaloric effect (MCE) of HoFeSi compound have been studied systematically. HoFeSi compound undergoes two successive magnetic phase transitions with the variation of temperature: a paramagnetic to ferromagnetic (FM) transition around TC =29 K followed by an FM to antiferromagnetic (AFM) or ferrimagnetic (FIM) transition at Tt = 20 K. The field dependence of magnetization reveals that a field-induced AFM/FIM-FM metamagnetic transition occurs below Tt with the increase in magnetic field. For a relatively low field change of 2 T, successive inverse and normal MCEs are observed and the maximum ΔSM values reach as high as 5.6 and 7.1 J/kg K around Tt and TC, respectively. This feature of successive inverse and normal MCEs in HoFeSi are suggested to be applied in some magnetic refrigerators with special designs and functions.