Research On Heat Convection Of Condensing Heat Exchanger

Condensing gas water heater has one more condensing system compared with common gas water heater.The water vapor in gas is condensed below the saturation temperature, then releasing latent heat. Exhaust gas temperature is reduced but thermal efficiency is increased. Acid gas such as CO2、 NOx and SO2is dissolved into the condensate water,so waste gas is purified.Then the purpose of energy saving and environmental protection is achieved.Condensing heat exchanger in condensing gas water heater, structure of helical flat tubes, is taken as the research object. Computational fluid dynamics(CFD) is used to simulate the flow fields. Pre-processing software Gmabit is used to build and mesh3D model of the heat exchanger.User defined function(UDF) of heat and mass transfer in water vapor condensation and multi-phase flow model in FLUENT are used to solve the problem of temperature distribution pressure distribution、velocity distribution and heat exchange coefficient heat transfer efficiency、 condensation effect. Experimental study is done on the thermal performance of condensing heat exchanger and its thermal efficiency is measured. Before numerical simulation, energy efficiency and the influence of gas properties and operating conditions on thermal performance are analysed.Then the method of handling on the thermal coupled wall、handling on water vapor condensation、setting method of boundary condition and material properties and solution method are introduced. Arrangement mode of flat tubes and tube materia、water inlet parameter gas inlet parameter the position of thermal baffle and are considerd to solve the influence on thermal performance of heat exchanger when numerical simulation. Flue structure optimization of the condensation section is made to gain high heat exchange coefficient.Study results show that Heat exchanger’s heat exchange coefficient of red copper tube and aluminium alloy tube are slightly higher than that of stainless steel tube.Through reducing wall thickness of stainless steel tube could get the same heat exchange coefficient of two others in the condition of safe wall thickness. And stainless steel tube takes reasonable purchase cost. Arrangement modes of flat tubes have a great effect on pressure drop between water inlet and water outlet,but almost nothing on thermal performance and condensation effect.Compared with water flow rate,inlet water temperature has more influence on thermal efficiency. Enlargement of inlet water flow and reduction of inlet water temperature result in reduction of the whole water temperature.So condensation area increases and more latent heat is released. Condensation rate and thermal efficiency are improved.When gas velocity increases, sensible heat exchange between gas and tubes weakens, flue gas loss holds more proportion of heat load,thermal efficiency drops and condensation effect declines.The position of thermal baffle changes heat exchange area of high temperature district and low temperature district. Gas flow also changes. Thermal performance and condensation effect are obviously affected by the position.Increasment of water vapour content enlarge its partial pressure,so condensation effect is better,the latent heat holds more proportion of heat load and thermal efficiency is improved obviously. Flue structure of the condensation section have an influence on heat exchange coefficient. When flue angle of chamfer is15°, there is no existing eddy stagnation region and heat exchange coefficient is higher.