An Investigation Of Heat Transfer And Flow Characteristics During Flow Boiling In Mini/micro-channels Under Hypergravity Environment

With the rapid increase of the power of the avionics on the modern high-performance fighter aircraft,the need of on-board vapor cycle cooling technology is more urgent.Since the fighter aircraft often encounter hypergravity environment during the maneuver flight,and the on-board vapor cycle cooling system often use the compact evaporator containing mini/micro-channels,it is necessary to investigate the heat transfer and flow characteristics during flow boiling in mini/micro-channels under hypergravity environment.For the above urgent requirement of on-board vapor cycle cooling technology,the heat transfer and flow characteristics of flow boiling in mini/micro-channels under hypergravity environment are experimentally and numerically investigated,and the calculation models for normal gravity and hypergravity are studied.The experimental investigation aims to obtain the heat transfer and flow characteristics of flow boiling in mini/micro-channels under hypergravity and the influences of hypergravity,inner diameter,mass flux,saturation pressure,heat flux and vapor quality on them.The numerical simulation aims to obtain the flow pattern of flow boiling in mini/micro-channels under hypergravity and explain the effect of hypergravity on flow boiling into mechanism.The investigation of calculation models for normal gravity and hypergravity aims to obtain the models with better predicting accuracy for normal gravity and propose new models for hypergravity.The main work and achievements are as follows:(1)An experimental system of flow boiling under hypergravity is designed and established,including hypergravity simulation platform,refrigerant loop and data acquisition system.An experimental investigation of R134 a flow boiling in 1.002,2.168 and 4.065 mm horizontal circular smooth copper tubes is conducted employing a centrifugal acceleration machine to simulate the hypergravity environment.The experimental conditions are mass flux of 185–935 kg/m2 s,heat flux of 18.0–36.0 kW/m2,saturation pressure of 0.578–0.82 MPa,vapor quality of 0.03–0.96,centrifugal acceleration of 0–3 g,and hyper-gravitational acceleration of 1–3.16 g.(2)Based on the normal gravity experimental data of heat transfer coefficient and frictional pressure drop,the effects of inner diameter,mass flux,saturation pressure,heat flux,and vapor quality are analyzed.The results indicate that the heat transfer coefficient under normal gravity decreases with increasing inner diameter,increases with increasing saturation pressure.Although the effects of mass flux and vapor quality on heat transfer coefficient are related to inner diameter,they are relatively weak in general,while the effect of heat flux on heat transfer coefficient is more significant,thus the flow boiling is dominated by nucleate boiling.The frictional pressure drop under normal gravity decreases with increasing inner diameter and saturation pressure,increases with increasing mass flux,basically maintain invariable with increasing heat flux,increases with increasing vapor quality until reaching the peak.(3)Based on the hypergravity experimental data of heat transfer coefficient and frictional pressure drop,the effects of hypergravity,inner diameter,mass flux,saturation pressure,heat flux,and vapor quality are analyzed.The results indicate that the deviations between heat transfer coefficient under hypergravity and that under normal gravity are basically within 50% for all tubes.For 1.002 mm tube,the heat transfer coefficient increases firstly and then decreases with increasing hypergravity,while for 2.168 and 4.065 mm tubes,it increases with increasing hypergravity all along.The deviations between frictional pressure drop under hypergravity and that under normal gravity are different for different tubes.For 1.002 and 2.168 mm tubes,the difference is basically within ±10% and the frictional pressure drop does not show specific rules with increasing hypergravity,while for 4.065 mm tube,the maximum difference might reach 300% and the frictional pressure drop increases with increasing hypergravity all along.Under a certain hypergravity,the effects of mass flux,saturation pressure,heat flux and vapor quality on heat transfer coefficient and frictional pressure drop are similar to those under normal gravity.(4)The calculation models of heat transfer coefficient,frictional pressure drop and void fraction during flow boiling under normal gravity are collected and organized thoroughly,and different libraries of calculation models are constituted respectively.Different libraries of experimental data which have wide experimental conditions and can be used for the evaluation of calculation models are established.Different calculation models are compared with the experimental data obtained from the present experiments and the collected data libraries respectively,and the models having better predicting abilities are yielded.(5)The applicability of the normal gravity models having better predicting abilities to hypergravity is analyzed.Based on the hypergravity experimental data of heat transfer coefficient and frictional pressure drop,and by introducing the effects of hypergravity and inner diameter,two models which can meet the design requirements of on-board vapor cycle cooling system of modern high-performance fighter aircraft are proposed by using the least square method.The new models compensate the week applicability of the models developed for normal gravity and improve the predicting accuracy.(6)The two-dimensional and three-dimensional numerical simulations of flow boiling in mini/micro-channels under hypergravity are conducted,by using the VOF model and the boiling heat transfer model compiled in UDF.The maximum hypergravity is 15 g during simulation,and the hypergravity orientation can be perpendicular to the flow direction or has the same or opposite directions with the flow at different conditions.According to the flow patterns obtained from the simulation,the effects of hypergravity and its orientation are analyzed.The results indicate that the effects of hypergravity and its orientation on flow patterns of flow boiling are significant.When the hypergravity orientation is perpendicular to the flow direction,the flow patterns evolve to stratified flow approximately with the increasing hypergravity,resulting in the dry out.When the hypergravity has the same direction with the flow,the bubbles are shortened,while their distances to the inner wall are not reduced.When the hypergravity has the opposite direction with the flow,the bubbles are elongated,their distances to the inner wall are increased,and the occurrence probability of dry out is reduced.The variation of flow patterns is the reason of the variation of flow boiling characteristics in mini/micro-channels under hypergravity.