Experimental Study of Horizontal Flow Boiling Heat Transfer Coefficient and Pressure Drop of R134a from Subcooled Liquid Region to Superheated Vapor Region

Tomáš Vít,Petra Dančová,Ernest Gyan Bediako

doi:10.3390/en15030681

Abstract

For the past few years, research in the field of flow boiling heat transfer has gained immense popularity for unravelling the dominant mechanism responsible for controlling heat transfer and identifying a parametric trend for understanding the characteristics of flow boiling heat transfer. This has led to several assumptions and models for predicting heat transfer during flow boiling without any known generalized mechanism. This study therefore seeks to experimentally study the characteristics of heat transfer during flow boiling over a wide range but small increase in vapor quality from a single-phase subcooled region through to a two-phase superheated vapor region. The study was performed with an R134a refrigerant in a single horizontal circular stainless-steel smooth tube that had an internal diameter of 5 mm. In this experimental study, local heat transfer coefficients and frictional pressure drop were measured for low heat fluxes of 4.6–8.5 kW/m2, mass fluxes of 200–300 kg/(m2s), vapor quality from −0.1 to 1.2 and a low constant saturation pressure of 460 kPa. Flow patterns observed during the study were recorded with a high-speed camera at 2000 fps. In covering a wide range of vapor quality, a peak of heat transfer coefficient near a vapor quality of zero and a local minimum observed in the low vapor quality region were observed, and both were sensitive to heat flux and mildly sensitive to mass flux. Generally, at low vapor quality, the heat transfer coefficient deteriorated with vapor quality and this was sensitive to heat flux but insensitive to mass flux and vapor quality, indicating nucleate boiling dominance in low vapor quality regions. In high vapor quality regions, the heat transfer coefficient was sensitive to mass flux and insensitive to heat flux. This indicates the dominance of convective boiling. In the low vapor quality regions, the flow patterns observed were slug and intermittent, while in the high vapor quality region, annular and mist flow patterns were observed. Generally, frictional pressure drop increased with increasing mass flux and vapor quality in the two-phase region.

Highlights

Research in the area of flow boiling heat transfer spans over decades [1,2]
There have been many studies on flow boiling heat transfer, with the majority of them focusing on the effect of mass flux, heat flux, vapor quality, tube diameter, refrigerant type and saturation conditions [5], there have been limited studies in the literature that have considered heat transfer characteristics from a subcooled liquid phase, through to two-phase, to a superheated vapor phase in a single experiment [6]
Extensive experiments performed over stored a wideto range but small increase in liquid and,quality causing a sharp rise in heat transfer coefficient

Summary

Introduction

Research in the area of flow boiling heat transfer spans over decades [1,2] This is because flow boiling heat transfer has been identified as an efficient technique for dissipating heat from small diameter tubes, which find their application in compact heat exchangers, air conditioners, refrigeration systems and many more [3,4]. There have been many studies on flow boiling heat transfer, with the majority of them focusing on the effect of mass flux, heat flux, vapor quality, tube diameter, refrigerant type and saturation conditions (saturation pressure and saturation temperature) [5], there have been limited studies in the literature that have considered heat transfer characteristics from a subcooled liquid phase, through to two-phase, to a superheated vapor phase in a single experiment [6] Many of these studies focus on heat fluxes greater than

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Conclusion