| Flooded evaporator is widely used large heat exchange equipment in refrigeration,energy and petrochemical industries.With the development of large public buildings in our country and the increasing requirement of energy conservation and emissions reduction,flooded evaporator has been widely used in large water chiller,heat pump and ice storage units.In addition,plain tube in flooded evaporator has been replaced with enhanced boiling tube with internal helically finned structure and external three-dimensional low fin structure,which greatly improves the thermal efficiency of the evaporator and performance of the unit.In recent years,with the development of variable flow technology by varying water pump frequency,the velocity of water flowing in tube also varies with the variation of operation condition of air conditioner,and the boiling condition outside the tube bundle also varies with supply temperature of chilled water or whether ice is stored or not.The above development also bring out some new issues.Specifically,firstly,due to the variable flow rate of chilled water in the air conditioning system,the Reynolds number in the tube of flooded evaporator is extended from 10000?40000 to 4000?40000.It is found that for the internal helically finned tube at a low Reynolds number especially in the range of 2300<Re<20000,universally recognized research conclusion still can't be drawn on the following aspects,such as the criterion of flow regime,determination of pressure drop and heat transfer rate,etc.Secondly,the evaporation temperature is in the range from-8 to 12? for the various types of flooded evaporators in air-conditioning refrigeration.However,for enhanced boiling tubes with three-dimensional low-ribbed outer surface structure,the following problems have not been well solved till now,such as the boiling heat transfer characteristics on single tube and tube bundle,determination of heat transfer rate,bundle effect etc.In addition,the study on pool boiling heat transfer outside enhanced tube bundle can be rarely found.At present,due to the lack of theoretical and experimental study on the above issues,the determination of flow and heat transfer characteristics in tube and boiling heat transfer performance on tube for various types of enhanced tubes can only rely on the manufacturers' "one-tube-one-test".The above test method has high test cost,large test error and can't draw universally prediction model,which seriously restrict the application of the test tube.Furthermore,this will restrict the design,development and innovation of flooded evaporator.To sum up,aiming at the above problems,for the enhanced tube with internal helical fin and external reentrant cavities structure which is widely used in flooded evaporator,systematic study will be conducted on flow and heat transfer characteristics in tube and pool boiling heat transfer characteristics on single tube and tube bundle.First of all,this article focuses on unsolved problems in the variation rule at low Reynolds number range of single-phase flow friction and convection heat transfer in the doubly enhanced tube boiling in the flooded evaporator.The research is conducted on experimental method for obtaining the flow friction factor and convection heat transfer coefficient in low Reynolds number range in flooded evaporator.Based on the analysis of experimental method,this article designs experimental system on the flow and heat transfer at low Reynolds number in tube under outside pool boiling condition.The construction of the experimental system is finished and the adjustment is also completed.This part of the work is meant to offer an experimental system with high precision and good stability for further study on the single-phase flow and heat transfer in the tube.Second,based on the above system,this article carries out experimental study on the friction factor and heat transfer coefficients at low Reynolds number in internal helically finned tube under pool boiling condition.The empirical models of friction factor and heat transfer coefficient are established.The variation rule of the critical Reynolds number for turbulent flow has been revealed to offer criterion for judging when the flow changes from transition to turbulent.It can be found that the proposed power exponential function could well describe the variation rule of the friction factor f and heat transfer coefficients j factor,the power function model of critical Reynolds number for turbulent flow has good prediction accuracy.Third,in order to study the pool boiling heat transfer characteristics on the enhanced tube with external reentrant cavity structure in the lower evaporation temperature,the thermal resistance separation method is adopted in this article to obtain the boiling heat transfer coefficient outside the tube,and the homologous error control method is adopted to guide the error control.And the design,processing and construction of the test system are completed.The experimental system shows good reliability and stability.Fourth,using the above test system,the R134a boiling heat transfer characteristics on the single tube with external reentrant cavities structure tube are experimentally studied within a given range of evaporation temperature and heat flux.Based on the experimental result,the below is studied,the variation relationship between boiling heat transfer coefficient with heat flux and evaporation temperature,the heat transfer enhancement factor of the enhanced tube,analysis on the effect of operating condition parameter on boiling heat transfer.Eventually,the empirical model of boiling heat transfer coefficient with operation condition parameters and refrigerant physical parameters is generated.The empirical model of boiling heat transfer coefficient for the tube with reentrant cavities structure does well in prediction accuracy.Fifth,bundle effect is the key problem to study and solve the pool boiling heat transfer on the enhanced tube bundle.Aiming at the problems such as the bundle effect and the lack of experimental study,the homologous experimental method for obtaining bundle effect is adopted to guide the experimental design and complete the design of the test section and control system,the processing and construction of the test system.The verification results show that the experimental system has excellent stability and good reliability.Sixth,with R134a boiling on the tube bundle,the bundle effect for the enhanced tube with external reentrant cavities structure is determined within a given range of evaporation temperature,heat flux and tube row number.Based on the experimental results,an empirical model is established for the relationship between the boiling bundle effect with heat flux,reduced pressure and tube row number.The results show that the empirical model for the bundle effect for the tube with reentrant cavities structure can predict the bundle effect for the test tube in the experimental condition with high accuracy. |
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