Study On Fluid Distribution In Plate Evaporator Based On Infrared Thermal Imaging

Plate heat exchangers are highly efficient and compact.They are widely used in food,chemical,metallurgy,HVAC and other fields.For plate heat exchangers used in refrigeration,the uniformity of fluid distribution is an important factor affecting heat transfer performance and system operation stability.For plate evaporator,the refrigerant at the inlet is usually in two-phase state,having a large velocity slip between gas and liquid.This can lead to obvious non-uniform distribution of flow rate,seriously affect the overall heat transfer performance,even cause local ice blockage and damage the heat exchanger.To solve this problem,presently the common practices adopted by the manufacturers are:(1)installing a distributor at the entrance of each plate to balance the flow between the channels;(2)designing diversion zone on the plate to make the fluid distribution in a single channel more uniform.The effects of the distributor or diversion zone are generally evaluated by testing the overall heat transfer performance of the heat exchanger.Due to the lack of information about fluid distribution in the channels,it is difficult to carry out targeted optimization design.In order to resolve the above problems,the following research work were carried out:1.A test bench for infrared monitoring of plate evaporator was built.On situ calibration was made for infrared temperature measurement.The influences of two factors,the annular distributor with small hole and the direction of fluid flow,were experimentally studied on the uniformity of fluid distribution in the heat exchangers.The infrared thermal images of side and end surfaces of the evaporators under stable working conditions were taken with an infrared camera.2.A processing method for thermal images of the corrugated surfaces of the plate evaporators was proposed.Through the algorithms of neighborhood average and substitution,the contact points and nearby areas in the images were smoothed.With the modified thermal images,the volume proportion of two-phase zone and the surface area of the phase boundary were estimated.An evaluation parameter for the uniformity of fluid distribution was put forward.For the case of staggered flow of secondary refrigerant,a mathematical model was established so as to estimate the flowrate distribution in a channel.3.According to the analysis of thermal images,the following results about fluid distribution were obtained:(1)Installing distributors increased the volume proportion of refrigerant two-phase zone in the plate evaporators and the overall heat transfer coefficient,and decreased the surface area of phase boundary.The improvement of fluid distribution uniformity brought by the distributors depended on the refrigerant flow rate.Within the range of tested refrigerant flow rate,the improvements of heat transfer showed strong positive correlation with the volume proportion of the two-phase zone.(2)For the single-phase flow of secondary refrigerant with inlet and outlet on the same side of the evaporator,the mass flow along the width direction of a plate tended to be high near the edges and to be low near the center.The heat transfer performance of co-current flow was better than that of counter-current flow,which could be reflected in the larger volume proportion of refrigerant two-phase zone.The fluid distribution in plate evaporators can be effectively estimated with the proposed method of infrared thermography and image processing.This method provides a new tool for plate heat exchanger manufacturers to improve product design and for refrigeration system manufacturers to evaluate the operations of plate evaporators.