Evaporation of refrigerants in a smooth horizontal tube:prediction of R22and R507heat transfer coefficients
百度在线翻译英语and pressure drop
教师节英语怎么说Adriana Greco a ,Giuppe Peter Vanoli
b,*
a
scandalsFacolt a di Ingegneria,Universit a degli Studi di Napoli FedericoII,Italy
b
Dipartimento di Ingegneria,Facolt a di Ingegneria,Universit a degli Studi del Sannio,Corso Garibaldi n.107,
Palazzo dell’Aquila Bosco Lucarelli,82100Benevento,Italy
Received 23July 2003;accepted 17February 2004
Available online 9April 2004
Abstract
This paper prents experimental heat transfer coefficients and pressure drop results obtained during the evaporation of pure R22and the azeotropic mixture R507(R125–R143a 50%/50%in weight).The test ction was a smooth,horizontal,stainless steel tube (6mm ID,6m length)uniformly heated by Joule effect.The effects of heat flux,mass flux and evaporation pressure on the heat transfer coefficients have been investigated.Each working parameter was bound within the range:evaporating pressure 3–12bar,refrigerant mass-flux 250–286kg/m 2s,heat flux 10.6–17.0kW/m 2,respectively.Additionally the experi-mental results have been compared with existing correlations which characterize the evaporative heat transfer coefficient to asss the validity of the models for refrigerant mixtures.Ó2004Elvier Ltd.All rights rerved.
Keywords:Heat transfer coefficients;Convective boiling;R22;R507;Smooth tube
1.Introduction
qingyouAs known the hydrofluorocarbons (HFCs)are a new family of substances harmless towards the ozone layer since they do not contain chlorine.
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Corresponding author.Tel.:+39-82-430-5577;fax:+39-82-430-5840.E-mail address:vanoli@unisannio.it (G.P.Vanoli).
1359-4311/$-e front matter Ó2004Elvier Ltd.All rights rerved.
doi:10.1016/j.applthermaleng.2004.02.006
Applied Thermal Engineering 24(2004)2189–2206
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Binary or ternary mixtures of HFC are often ud in lieu of pure fluids since,by mixing two or three components,the overall,required properties can be obtained.
Experimental tests carried out with the new refrigerants have pointed out a reduction of the energetic performances of the systems that means a larger environmental impact in terms of ‘‘greenhou effect’’.Accurate investigations,either on the overall system either on single device of the plant,highlighted how the performance are influenced from the functioning of the heat exchangers (evaporator and condenr).
For this reason the knowledge of two-pha flow heat transfer coefficients is of prime impor-tance to optimi the design of heat exchangers for refrigeration and air conditioning applications.This point is most important for HFC’s mixtures becau at prent many aspects of boiling and condensing heat-transfer with pure components are well known and reasonably good cor-relations have been developed for the design of efficient heat exchange equipment while with mixtures,boiling and condensing heat-transfer is more complicated.
Knowing that in the real applications the refrigerant is mixed with the lubricant oil of the compressor,which could reduce the heat transfer coefficient,another important field of rearch is the study of the heat transfer coefficient of the refrigerant-lubricant oil mixture.
A vast amount of rearch has been carried-out on flow-boiling of pure,mixed refrigerants and of refrigerant–lubricating-oil mixtures.Table 1summarizes the most relevant works on the subject.In the majority of flow boiling tests,local boiling heat-transfer coefficients are expresd as a function of vapour quality rather than of mean values over a whole tube with a large change in vapour quality or even exit superheating.In-tube evaporation is utilized in direct expansions evaporators with horizontal tubes and all but one of the studies ud this tube
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2190 A.Greco,G.P.Vanoli /Applied Thermal Engineering 24(2004)2189–2206
A.Greco,G.P.Vanoli/Applied Thermal Engineering24(2004)2189–22062193 where the complicating effect offlow stratification has to be confronted.The evaporative coeffi-cient were evaluated for smooth and micro-fin tubes of different internal diameter.
Many data were obtained using electrically heated test ctions.For test conditions with complete the internal perimeter of the evaporator tube,this is a reasonable approxi-mation of a uniform heat-flux boundary-condition.Other test data were obtained using counter-currentflow of hot water to evaporate the refrigerant.In this ca,the boundary condition is clo to uniform wall-temperature.
After this bibliographic analysis the rearch group designed and built an experimental apparatus in order to develop a detailed analysis of the characteristic of the heat transfer of pure and mixed refrigerants in a great range of evaporating pressure and massflux.The test apparatus built at University of Naples prents a test ction electrically heated.So it is possible to evaluate the local heat transfer coefficients and pressure drop during convective boiling.The results pre-nted in this report concern the evaporation of pure R22and of the azeotropic mixture R507 (R125–R143a50%/50%in weight).The objective of the prent experimental study are to: (i)develop an accurateflow boiling heat transfer databa for veral important newfluids, (ii)provide data to the refrigeration industry for the design of high efficiency evaporators, (iii)compare the thermal performadml
nces of R507to R22and(iv)investigate the influence of the evaporating pressure and of the heatflux on theflow boiling characteristics of thefluids.
爱情公寓第三季发布会2.Experiments
2.1.Experimental apparatus
The experimental plant was designed to investigate two-pha-flow heat-transfer phenomena during convective boiling under uniform heat-flux conditions.
A schematic view of the experimental apparatus is provided in Fig.1.It consists in two main loops:the refrigerant loop and a condary vapour-compression loop for refrigerant-fluid con-densation.
The refrigerantfluid evaporates in the test ction.Its condensation is achieved in a plate type evaporator.A gear pump drives the refrigerantfluid circulating in the experimental loop.
The pump is equipped with a hydrodynamic speed variator allowing experimental test at dif-ferent refrigeratingfluidflow-rates.In this way the massflux can be varied within the range200–1100kg/m2s.In the main loop,the refrigerantfluid is entirely oil-free.Usually in a typical vapour compression plant the refrigerantfluids contains approximately2–4%by weight of compressor lubricat
ing oil in dependence of the lay-out of the plant and of the refrigerant temperature level. The experimental t-up,however,enables the injection of predetermined amounts of lubricant oil into the refrigerantfluid,thus extending the experimental measurements to more realistic oper-ating conditions,as well.
account是什么意思The evaporation of testfluid occurs in6m stainless steel horizontal tube,inside diameter6mm, wall thickness1mm.The energy required for evaporation is provided by Joule effect.A direct electrical current supplied by a feed current device circulates within the wall of the tube.The heat flux can be safely assumed to be constant along the tube length.
