Fundamental Researches Of Gravity Film Heat Exchanger And Its Application In Hybrid Source Heat Pump System

Energy recycling and waste energy recovery are recognized as generalized renewableenergy as energy crisis is more and more serious. Gravity film heat exchanger (GFX) is anew type of heat exchanger which recovers waste energy from waste water. Normally, GFXis composed by vertical central drain pipe and narrow spiral tube which surrounds theoutside of drain pipe, and the GFX is located in the drainage pipe to extract residual heatfrom industrial or domestic waste water. The researches on GFX are not so good, so it isnecessary to lucubrate GFX technology to facilitate its utilization.The flow characteristics, heat exchange characteristics, theoretical efficiency, practicalefficiency of GFX are researched based on theoretical analysis, experimental verificationand numerical simulation accordingly. The researches include: flow characteristics ofgravity film in GFX central drain pipe, and the correlation between flow characteristics andheat transfer characteristics; numerical description of gas-fluid flow characteristics of airengulfment in vertical drain pipe and water flow converge in the pipe; modeling on externalspiral pipe of GFX and its flow characteristics; analysis model of GFX efficiency;performance analysis of hybrid solar source heat pump with GFX.Gravity film flow characteristics of the central pipe in GFX are analyzed byexperimental research and theoretical analysis. The experimental bench of GFX verticalcentral drain pipe gravity film flow characteristics is set up, and the bench also consists of ahigh-speed CCD photography which monitors the film characteristics. This bench will notdisturb the flow field, and it is benefit for observation of the micro-scale features. Theexperiments focus on the flow characteristics of the free gravity film flow in the centraldrain pipe, and the effects of fluid viscosity and pipe diameter on the flow pattern. Theresults show that there are three types of flow pattern: spiral flow, film flow and choke flow.There is limit length, limit thickness, and limit velocity to form the whole film. There iscritical Reynolds when the spiral flow changes into film flow. On the surface of the gravityfilm there is complicated fluctuation, and changing characteristic in space and time isrelative to Reynolds and the scale of vertical distance. As the increment of Reynolds, theaverage film thickness will be increased, and fluctuation will be intensified. If the Reynolds is constant, the fluctuation will be intensified by larger vertical distance. The experimentalresults from three different fluids indicate that higher viscosity will increase the thicknessof gravity film, and weaken the fluctuation。 The fluctuation of the gravity film in twodifferent diameter pipes is very similar, the critical Reynolds at which spiral flow changeinto film is larger in smaller diameter pipe than in larger diameter pipe. The limit length,limit thickness and limit velocity is solved by theoretical analysis.The flow characteristics and heat exchange characteristics of gravity film in GFXcentral drain pipe are researched by theoretical analysis. The results indicate the heatexchange affected by film thickness, fluctuation, secondary return flow in the film. Theheat transfer efficiency of gravity film is affected by the conduction in the laminar flow andconvection in the turbulent flow. The heat exchange rate in the laminar flow depends onfilm thickness, while the heat exchange rate in the turbulent flow depends on film basethickness and turbulent characteristic.The extended researches on the factors of central drain pipe gravity film are conductedaccording to the numerical simulation. The results indicate that The increment of inletvelocity of the gravity film will intensify the fluctuation of fluid film, and will eliminatebroken of gravity film. The smaller velocity difference between gas and fluid will inhibitfilm broken. The thicker gravity inlet film will translate the broken location to the outletdirection, while the thicker film will intensify the broken phenomenon.The flow characteristic in the external spiral pipe is studied by numerical simulation.The maximum velocity, velocity field and secondary return flow in spiral pipe are analyzedbased on round, oval and octagonal section spiral pipes. The dimensionless parameter"relative secondary velocity", the ratio of area averaged secondary return flow to areaaveraged main flow, is defined to compare the secondary return flow and main flow. Theanalysis method can be used to optimize and distinguish the section of external pipe inGFX.The efficiency of the GFX is analyzed by the theoretical model which is verified bythe experiments. The results indicate that the flow ratio of hot fluid and cold fluid (R),diameters of internal pipe and external pipe of the heat exchanger will significantly affect the efficiency of GFX. There is optimized flow ratio (R), diameters. And there is optimizednumber of external tubes for certain flow ratio.The performance of the hybrid source heat pump with GFX is analyzed by Simulink.The results indicate the efficiency of the system is significantly improved by the GFXwhich recovers residual heat from waste water. And the method to recover residual heat issignificant for the system performance. The results will be benefit for sewage heat pumpwhich consists of GFX.In summary, the heat exchange process in GFX is very complicated. The results onflow and heat exchange mechanism of central drain pipe and external spiral tubes whichbased on the theoretical analysis, experimental research, and numerical simulation aresignificant for widespread application of GFX.