Study On Friction And Convective Boiling Heat Transfer Of R134a In Helically-coiled Tubes

Helically coiled tubes which are compact in structure especially laid horizontally,are widely used in galley,submarine and space station et al which need narrow space to act.And due to function of secondary flow in helically coiled tubes produced by centrifugal force,this kind of tubes is more efficient in heat transfer than straight tubes.However,by now majority of investigation on flow boiling heat transfer in helically coiled tubes considered vapor-water as working fluid,and more attention were paid to critical heat flux,post dry out heat transfer phenomenon et al.Studies of R134a flow boiling heat transfer in horizontal helically coiled tubes are less reported.When helically coiled tubes are laid horizontally,partial two-phase flow situation in tubes is more complex than that in straight tubes because of the angle between centrifugal force and gravitation is continuously changing.Experiential related formulae and models foregone were needed to be modified and extended to more wide application range,and it is necessary to study deeply on the characteristics of flow boiling heat transfer in horizontal helically coiled tubes.Alternative refrigerant R134a was used as working fluid to investigate the characteristics of flow boiling heat transfer and resistance in horizontal helically coiled tubes in this paper.Mechanism of heat transfer and resistance also were analyzed and the experimental parameters were:mass flux G=150～550kg/m~2s, heat flux q=2.0～20kw/m~2,flow quality x=0.1～0.85.The main works in this paper were as follows:(1) Experiment structure is the basis of the article.First,process of the experimental system was designed.In equipment selection and purchase process, all the necessary equipment was carry out a wide range of research.Commission-processing the experimental and preheated section,sheathed thermocouples, transparent spiral experimental section and various non-standard fittings. Self-processing the copper-constantan thermocouple to measure the wall temperature,and through high-precision constant temperature water bath to check each thermocouple.After the completion of system carry out air tightness, pressure and vacuum tests repeatedly,until reach the test requirements.(2) Visual experiments were performed to observe the flow patterns in horizontal helically coiled tubes using transparent nylon experiment sections. Several kinds of flow patterns were screened for comparing and analysis such as bubble flow,plug flow,slug flow,stratified flow and annular flow.(3) Experiments were carried out to study the regulation of partial heat transfer coefficient versus the temperature following tube length and tube circle sections,mass flux and heat flux.And the results were compared with those in horizontal straight tubes.(4) Experiments were carried out to study the regulation of pressure drop versus the mass flux and saturation pressure.And the results were compared with those in horizontal straight tubes.Based on the experimental data and analysis, relations between liquid friction factor and Martinelli parameter and new experiential formula were developed.(5) Numerical simulation was also completed in this study for horizontal helically coiled tubes of five kinds of different structure parameters using R134a as modeling fluid(single liquid phase flow).Temperature and velocity distribution were obtained to analyze the heat transfer coefficient vs.the different structure parameters.Provide a reference for future design and optimization horizontal spiral heat exchanger.