CONDENSATION HEAT TRANSFER OF R-134a INSIDE HELICALLY COILED HORIZONTAL TUBES

Abstract

An experimental exploration has been carried out to investigate the enhancement in heat transfer coefficient and pressure drop inside helically coiled tubes over the straight ones during condensation of the pure vapor of R-134a. For this purpose, coiled tubes with different geometries viz. different curvature ratios at constant pitch and different pitches at constant curvature ratio have been tested. Experiments were also carried out in straight tube to generate baseline data for comparison with helically coiled tubes. All the test-sections, helically coiled tube-in-shell and straight tube-in-tube were positioned horizontally in the experimental set-up. In helically coiled tube-in-shell test-condenser, helically coiled copper tube was placed concentrically in annulus shell. The helically coiled tube with varying curvature ratio (δ = 0.0917, 0.0593 and 0.0395) at constant non-dimensional pitch (λ = 0.08) and with varying non-dimensional pitch (λ = 0.08, 0.158 and 0.237) at constant curvature ratio (δ = 0.0917) were fabricated from soft copper tube of inner diameter 8.33 mm and straight length varying between 2000 to 3480 mm. The dimension of the shell varied with different geometry of the coiled tube. The straight tube-in-tube test-condenser had a hard drawn copper tube having 8.33 mm inside diameter and 1000 mm in length, fixed concentrically inside a copper pipe with 43 mm inner diameter. The refrigerant was condensed inside the internal tube and the coolant water was passed through the annular space of both type of test-condenser in counter flow direction. The refrigerant mass flux was measure with a Coriolis effect flow meter and the flow rate of cooling water in the test-condenser was measured with a turbine flow meter. The temperature on the outer wall of the inner tube of test-condenser was measured using T-type thermocouples at four axial locations in straight tube and at six locations on coiled tube. The thermocouples were soldered at the top, bottom, left and right sides positions at each location. The gain in temperature of coolant water in test-condenser was measured with the help of thermopiles. The pressure and the temperature of refrigerant were measured at the entry of test-condenser, and at the inlet and outlet of steel tube pre-heater as well. Pressure gauges were also fixed at the inlet and the outlet header of the bank of three refrigerant pumps to facilitate the monitoring of pressure in the refrigerant loop of the experimental set-up. The refrigerant mass flux was regulated by adjusting speed of pump through a frequency controller. The heat input to pre-heater was supplied through an auto-transformer for the iv desired quality of vapor at the entrance of test-condenser. The experimental data were acquired for eleven different refrigerant mass fluxes in the range of 100 to 600 kg m-2 s-1 in an interval of 50 kg m-2 s-1 Initially, the integrity of the experimental set-up was ascertained by acquiring the data for the condensation of R-134a inside a straight tube. These data were evaluated with the predictions from different widely cited correlations. The experimental data of heat transfer coefficients were evaluated against the predictions of different correlations and the data were in the best agreement with Shah (1979) correlation. The pressure drop data were also predicted by the correlations of other investigators and the best agreement found with the correlation of Choi et al. (1999) correlation. . Almost 10 test-runs were carried-out for each mass flux so as to cover the entire range of vapor quality from 0.1 to 0.9. A total of five helically coiled tubes with different curvature ratios and pitches were investigated and a straight tube was also used to compare the condensation performance of coiled tubes. The average heat transfer coefficients, average pressure drop and average vapor quality were calculated for each test-condenser and for each test-run. The deviation in heat transfer coefficients as well as pressure drop with vapor quality, saturation temperature and refrigerant mass flux has been studied. The flow regime studies have also been carried out for helically coiled tubes. After the integration of experimental set-up was established, experiments were carried out with helically coiled tubes with different geometries. For the coiled tubes, the impact of curvature ratio varying from 0.0917 to 0.0395 at fixed dimensionless pitch of 0.08 on condensation performance of R-134a is remarkable. The highest heat transfer coefficient as well as pressure drop has been attained for the highest coil curvature ratio of 0.0917 with pitch value of 0.08 for the entire range of refrigerant mass flux. The influence of variation of dimensionless pitch from 0.237 to 0.08 at fixed curvature ratio of 0.0917 is not significant on both heat transfer coefficient and pressure drop. Both these characteristics increase marginally with increase in pitch of the coil at fixed curvature ratio. With rise in vapor quality and refrigerant mass flux, improvement in heat transfer coefficient and rise of pressure drop in all the helical tubes have been noted. v It is observed that all coiled tubes provide enhancement in heat transfer coefficient and increment in pressure loss compared to straight tube at each mass flux. The coiled tube of highest curvature ratio of 0.0917 with highest dimensionless pitch 0.237, gives highest enhancement in heat transfer and penalty in press drop for entire range of mass flux. The enhancement factor decreases as mass flux increases for all the coiled tubes. However, the penalty pressure drop factor enlarged with the rise of refrigerant mass flux. For all the coiled tubes the heat transfer enhancement is more in high vapor quality region as compared to low vapor quality region, however at the same time, the pressure drop penalty is also high. Several correlations have been used to compare the experimental data of helically coiled tubes. The best agreement of data has been found with the heat transfer coefficient and pressure drop correlations of Wongwises and Polsongkram (2006) proposed for helically coiled tube-in-tube heat exchanger. The following empirical correlations have been developed for helically coiled tube: Heat transfer coefficient 626.004.0022.033.074.0Pr148.0−λχ=rtteqtppDeNu Two-phase frictional pressure drop multiplier 35.0063.0164.02376.121613.61078.1−λδ  χ+χ+=φrttttlp Two-phase friction factor 863.0041.0348.0284.0Re24.0−λδ=reqtppf During the experiments, visual examination of two-phase flow patterns were carried out by viewing through the sight glass placed after the test-condenser. A set of phases consist of stratified-wavy, wavy-annular, semi-annular annular and mist-annular flow has been observed in the current investigation. A flow regime map based on liquid and vapor Weber number has been developed and transition between different flow regimes is also on it. An early transition of flow structure to annular was observed due to secondary flow generated in the helical coil tubes. The annular flow occupied larger area of the flow regime map. The stratified-wavy flow regime is observed for Weber number values lower than 7.22 for liquid phase and 65.5 for vapor phase. The transition from wavy-annular to semi-annular flow regime follows following Equation: vi 488.011.14lvWeWe=, for 12 ≤ Wel The transition line between semi-annular and annular flow regime has been given as: ≤ 238 661.03035.19lvWeWe=, for 9.6 ≤ Wel ≤ 219 The mist-annular was observed for vapor Weber number above 3800.