Condensation and evaporation heat transfer of R410A inside internally grooved horizontal tubesTransfert de chaleur à la condensation et à l'évaporation, pour R410A à l'intérieur des tubes rainurés horizontaux

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Abstract

Heat transfer coefficient and pressure drop were measured for condensation and evaporation of R410A and HCFC22 inside internally grooved tubes. The experiments were performed for a conventional spiral groove tube of 8.01 mm o.d. and 7.30 mm mean i.d., and a herring-born groove tube of 8.00 mm o.d. and 7.24 mm mean i.d. To measure the local heat transfer coefficients and pressure drop, the test section was subdivided into four small sections having 2 m working length. The ranges of refrigerant mass flow density was from 200 to 340 kg/(m2 s) for both condensation and evaporation of R410A and HCFC22, and the vapour pressure was 2.41 MPa for condensation and 1.09 MPa for the evaporation of R410A. The obtained heat transfer data for R410A and HCFC22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of spiral one for condensation and are slightly larger than those of spiral one for the evaporation. The measured local pressure drop in both condensation and evaporation is well correlated with the empirical equation proposed by the authors.

Résumé

Les auteurs ont mesuré le coefficient de transfert de chaleur et la chute de pression lors de la condensation et l'évaporation de R410A et de HCFC22 à l'intérieur de tubes rainurés horizontaux. Les expériences ont été effectuées sur un tube spirale rainuré classique d'un diamètre extérieur de 8,01 mm, un diamètre interne moyen de 7,30 mm et un tube rainuré à chevrons avec un diamètre extérieur de 8,00 mm et un diamètre interne moyen de 7,24 mm. Afin de mesurer les coefficients de transfert de chaleur locaux et la chute de pression, la section d'essai a été sous divisée en quatre petites sections chacune d'une longueur de 2 m. Le débit massique était de 200 à 340 kg/(m2s) pour la condensation et l'évaporation de R410A et de HCFC22, et la pression de la vapeur était de 4,41 MPa lors de la condensation et de 1,09 lors de l'évaporation de R410A. Les données sur le transfert de chaleur de R410A et de HCFC22 indiquent que les valeurs des coefficient de transfert de chaleur du tube rainuré à chevrons sont environ deux fois celles du tube à spirale en ce qui concerne la condensation et sont légèrement supérieures à celles du tube à spirale lorsqu'on considère l'évaporation. La chute de pression locale mesurée lors de la condensation et de l'évaporation reflète l'équation empirique proposée par les auteurs.

Introduction

In recent years, many kinds of internally grooved tube have been developed to improve the performance of air-conditioning heat exchanger for HFC refrigerants and the experimental studies of the heat transfer characteristics of those tubes have been carried out [1]. Recently, Cavallini et al. [2] reviewed the many works concerned with the condensation heat transfer and pressure drop of these tubes. They compared the many experimental data published in literatures with previous correlations, and revealed the influence of compressor oil on condensation of refrigerants inside enhanced tubes. For the evaporation heat transfer, the authors [3] reviewed the proposed correlation which can apply to internally grooved tubes for HCFC22, R410A and R407C and suggested that the effect of the shape of micro-fin on heat transfer enhancement was not revealed well yet. Regarding the characteristics of pressure drop inside a tube, the empirical equations for smooth tube were proposed by [4], [5], [6] and those for micro-fin tube were proposed by [7], [8]. However, for the different kinds of internally grooved tube, it is difficult to estimate exactly pressure drop from these equations during both condensation and evaporation in heat exchanger. The purpose of this study was to experimentally obtain the characteristics of the pressure drop inside two different kinds of internally grooved tube with different micro-fin shape for HCFC22 and R410A, and also to experimentally verify the previous proposed equation for heat transfer coefficients by the experimental data that were taken at the same time when the pressure drop was measured.

Section snippets

Experimental apparatus and procedure

Fig. 1 shows a schematic diagram of the experimental apparatus, which is a vapour compression heat pump system, made up of a refrigerant-loop and two water-loops. The refrigerant-loop consists of a compressor (1), an oil separator (2), a four-way valve (4), a condenser (6), a mass flow meter (8), an expansion valve (10), an evaporator (11) and an accumulator (12). Each of the water loop is composed of a constant temperature water tank (15 or 16), a line pump (14), a flow control valve, a

Data reduction

On the reduction of the experimental data, the local evaporation and condensation heat transfer coefficient α is defined as:α=QAreal±Twi∓Trwhere Q is the heat transfer rate at each section which was obtained from the heat balance for the cooling/heating water flowing inside the annulus and Areal is real heat transfer surface area of internally grooved tube. Twi and Tr represent the arithmetic mean temperature of each section at the inner surface of the inside tube and the arithmetic mean

Local evaporation heat transfer coefficient

The measured local evaporation heat transfer coefficients α for HCFC22 and R410A are shown in Fig. 4(a) and (b) in terms of dimensionless form ααL, where αL is the heat transfer coefficient of liquid component flowing alone inside a smooth tube which was calculated from the Dittus–Boelter equation [10] and x is vapour quality. The symbols ○ and • denote the measured data of HCFC22 for C tube and W tube, and the symbol ▴ denotes those of R410A for W tube, respectively. The solid and the dotted

Conclusions

An experimental study of evaporation and condensation heat transfer using two kinds of internally grooved horizontal tubes for HCFC22 and R410A were carried out.

The effect of shape differences on heat transfer enhancement between herring-bone grooved tube and the conventional grooved one are shown. The obtained heat transfer data indicates that the herring-bone grooved tube is more effective in enhancing evaporation and condensation heat transfer than the conventional spiral groove tube. It is

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