The Research Of Two-phase Flow Boiling Model In Cooling Passage Of Diesel Engine

Based on the simplified rectangular channel and a turbocharged diesel engine,the study of two-phase flow boiling model has been carried out in cooling passage.The effects of coolant flow rate,heating surface temperature,system operating pressure and coolant temperature on boiling heat transfer process are analyzed through K Robinson rectangular channel experimental rig.The computational error between two-phase&single-phase flow boiling model of the rectangular channel will be compared.The experimental rig of diesel engine cylinder head was setup to measure the temperature.When the diesel engine runs calibrated condition,the temperature of key positions were measured by using hardness plug.The computational error of measuring points between two-phase flow boiling model and forced convection model in practical diesel engine cooling system were investigated through CFD software and fluid-solid coupling method.The distribution condition of temperature field,coolant streamline and coolant flow field were investigated.The main work and conclusions are summarized.(1)The two-phase flow boiling model includes two-phase flow model and wall boiling model.The computational accuracy,application range and grid type of three kinds of two-phase flow boiling model(VOF model and superposition method,Mixture model and superposition method,Euler model and mechanism method)were compared.For comprehensive consideration of computational accuracy and time cost of model,the two-phase flow boiling model was developed by using two-phase flow model and wall boiling model.The heat transfer characteristics of different model in simplified rectangular channel and turbocharged diesel engine were studied.(2)Taking experimental data of K Robinson rectangular channel as the reference,the channel model was constructed according to the proportion of 1:1,and the 10 x 16x 241 grid scheme was selected through the study of grid independence.The numerical schemes of the different factors on heating surface heat flux were made,and the factor include coolant flow temperature(85,90&95?),heating surface temperature(90?~160?),coolant flow rate(0.25,0.5&1 m/s),operating pressure(1,2&0.3 MPa).The numerical schemes were calculated by using two-phase flow boiling model and single-phase flow boiling model,and the total number of numerical schemes is 200.The results show that when the heating surface temperature rises,the wall heat flux will increase.While the coolant flow rate increases and coolant flow temperature decreases,the wall heat flux will increase.The smaller system operating pressure is,the higher heating surface heat flux is.The calculation results of two kinds of models is consistent with the experimental data.The heating surface average heat flux error between two-phase flow boiling model and experimental results is less than10%,and the error of single-phase flow boiling model exceeds 30%.The two-phase flow model can improve the calculation accuracy,and reflect the distribution of gas-liquid two-phase flow in the passages.(3)According to the heat transfer progress in practical diesel engine,the twenty hardness plugs were arranged to measure the temperature of cylinder high load area in calibration condition.The experimental results show that the maximum temperature of cylinder head from first cylinder head to fourth cylinder head is respectively 562 K,561 K,580 K and 574 K,and the temperature of third and fourth cylinder is higher than that of first and second cylinder.Measuring points 2,9 and 20 are located in the same position,and the tempeature of three measuring points are 555 K,555 K and514 K.The point 20 is 41 K lower than that of other points.Considering experimental error and data rationality,the measuring points 20 is decided to remove.For the same cylinder head fire cycle the temperature of bridge zone between exhaust valves is the highest,and the temperature of bridge zone between exhaust valve and intake valve is the second,and the temperature of bridge zone between intake valves is lowest.(4)Taking diesel engine cooling system as research object,the grid model and geometry model was constructed,and two-phase flow boiling model and forced convection heat transfer model were used to calculate the heat transfer process.The maximum flow velocity in cooling passage is 12.7 m/s,and the steam line of the inflow side is denser than the back flow side.The local minimum velocity of cylinder head is 0.38 m/s.For two-phase flow boiling model and convection heat transfer model,the maximum temperature of cylinder head firecycle is 419 K and 429 K,and the maximum temperature of cooling passages is 577.4 K and 584.4 K.The maximum error and average error of the calculated value of forced convection heat transfer model is 5.32%and 2.54%.The maximum error and average error of the calculated value of the two-phase flow boiling model is 3.83%and 1.75%.The bubble of third cylinder and fourth cylinder are distributed along the intake and exhaust passage.The largest vapor volume fraction is 0.18,and the average vapor volume fraction in outside of first and second cylinder intake passage is 0.11.The maximum heating wall heat flux of forced convection heat transfer model and two-phase flow boiling model is 8.3×10~5 W/m~2 and 9.3×10~5 W/m~2.