Investigation of R134a Flow Boiling Heat Transfer and Pressure Drop in the Evaporator Test Section of Refrigeration System

Zahraa Kareem Yasser,Ahmed J. Hamad

doi:10.31026/j.eng.2019.01.02

Abstract

This paper presents an experimental and theoretical analysis to investigate the two-phase flow boiling heat transfer coefficient and pressure drop of the refrigerant R-134a in the evaporator test section of the refrigeration system under different operating conditions. The test conditions considered are, for heat flux (13.7-36.6) kW/m2, mass flux (52-105) kg/m2.s, vapor quality (0.2-1) and saturation temperature (-15 to -3.7) ˚C. Experiments were carried out using a test rig for a 310W capacity refrigeration system, which is designed and constructed in the current work. Investigating of the experimental results has revealed that, the enhancement in local heat transfer coefficient for relatively higher heat flux 36.6 kW/m2 was about 38% compared to 13.7 kW/m2 at constant operating conditions. The enhancement in heat transfer coefficient was about 57% when the mass flux increased from 52 kg/m2.s to 105 kg/m2.s at constant test conditions. The enhancement in the heat transfer coefficient was about 64% when the saturation temperature increased from -8 to -3.7 at fixed refrigerant mass velocity and heat flux. The effect of mass velocity on pressure drop was relatively higher by about 27% than that for heat flux at specified test conditions. The comparison between the experimental and theoretical results has shown an acceptable agreement with an average deviation of 21%.

Highlights

Boiling heat transfer is an effective mode of heat transfer that happens with a change in phase from liquid to vapor
A noticeable reduction in heat transfer coefficient was observed at higher vapor quality (x > 0.8) due to the dry out the effect of refrigerant boiling in the evaporator tube which occurs at relatively higher heat flux and low mass velocity
The behavior of the local heat transfer coefficient and pressure drop were investigated and the following conclusions can be derived: 1-The heat flux applied to the test section has a significant effect on refrigerant R-134a flow boiling heat transfer coefficient

Summary

Introduction

Boiling heat transfer is an effective mode of heat transfer that happens with a change in phase from liquid to vapor. Park and Hrnjak, 2007 studied the boiling heat transfer coefficient and pressure drop in a horizontal smooth copper tube of 6.1 mm inner diameter for (CO2, R410A, and R22) at mass flux from (100 to 400) kg/m2.s, evaporation temperatures of -15 oC and -30 oC and heat flux from (5 to 15) kW /m2 for vapor qualities ranging from (0.1 to 0.8). Del Col, et al, 2010 conducted an experimental investigation of flow boiling heat transfer for R134a, R125, R22 and R410A in 8 mm inner diameter horizontal tube with a mass flux of 200–600 kg/m2.s, heat flux 9–53 kW/m2, saturation temperature 25–45oC, and vapor quality ranging from 0.07 to 0.87. A modified heat transfer coefficient correlation for alternative and natural refrigerants was proposed and predicted well the experimental data

Methods

Results

Conclusion