Simulation Study On The Effect Of Coolant On Boiling Heat Transfer In The Cooling Water Jacket Of Internal Combustion Engines

As the emission regulations require,the internal combustion（IC）engine gradually develops to high power,compact and lightweight,which leads to the severe heat load problem of the IC engine.Therefore,the heat transfer capacity of cooling system needs to be strengthened.The use of coolant with higher heat transfer capacity has become the research direction of many scholars,and the wall temperature near the bridge passages areas of cooling water jacket is higher than the saturation temperature of coolant,so the boiling heat transfer exists in cooling system.In this paper,two-phase flow model and three-phase flow model of subcooled boiling heat transfer are used to simulate the boiling heat transfer process of pure water and nanofluids in cooling water jacket,respectively.The heat transfer process and mechanism of pure water and nanofluids are studied based on the data of heat flux,pressure,velocity and bubble volume fraction.The main research contents and conclusions are summarized as follows:（1）Based on Euler-Euler model,a two-phase flow model of subcooled boiling heat transfer is established.On this basis,a three-phase flow model is established by coupling Euler-Lagrange model.Two-phase flow and three-phase flow model are used to simulate boiling heat transfer process in cooling water jacket of the cylinder head with pure water and nanofluids as coolants,respectively.The accuracy of both models is verified by the simplified circular channel test data of Kim SJ.It is found that the average error between simulation values and test values of both models is less than 5%,which has good calculation accuracy.The effects of concentration and velocity on boiling heat transfer of nanofluids were investigated.The results show that the heat transfer coefficient increases with increasing concentration of nanofluids,decreases first and then increases along the flow direction,and decreases with the increase of velocity.（2）The subcooled boiling heat transfer process in the cooling water jacket of the cylinder head is numerically simulated.According to the requirements of numerical simulation,the geometric model of cylinder head is established.The interface mesh is shared according to the simulated conditions,and the solid mesh and fluid mesh are established separately.On the basis of technical parameters and working conditions of IC engine,the flow boundary conditions such as entrance and exit and the thermal boundary conditions of wall are determined.（3）Compared simulation results with pure water and Al₂O₃-water nanofluids,the heat flux increases with increasing concentration of nanofluids,and the average heat flux of cooling water jacket wall increases by 9.34%when the volume fraction of nanofluid is 2.0%,which proves that nanofluids have better heat transfer ability.The distribution of heat flux is observed,and the composition of heat flux is analyzed to determine that the increase of quenching heat flux contributed greatly to heat transfer enhancement.The maximum pressure drop is 17.2 kPa,and the pressure distribution in cooling water jacket is uniform and the pressure drop between cylinders is small.Therefore,the pump power consumed by the coolant flowing through the cooling water jacket is small.The pressure drop of nanofluids coolant increased slightly,but the pressure distribution trend is basically same,and the maximum pressure drop of 2.0%volume fraction nanofluid is 2.3 kPa higher than that of pure water.Through the analysis of heat flux and pressure drop,it is concluded that nanofluids are beneficial to enhance the heat transfer of cooling system with small pump power loss.（4）The applicability of the two-phase flow model is determined by comparing the simulated and experimental measuring point temperatures of firecycle with pure water as coolant.It is found that the temperature of 20th measuring point at the area of fourth cylinder intake port and injector is contrary to that of other measuring points,so it should not be discussed.Secondly,the average error of two-phase flow simulation values and experimental values is only 1.743%.Therefore,the two-phase flow model has good applicability.There is a big gradient at the cylinder head temperature,the highest temperature is 576.4 K,which is located at the bridge passage of fourth cylinder two exhaust ports.The wall temperature of cooling water jacket near the side of the firecycle is higher than other regions,and shows an upward trend from the first cylinder to the fourth cylinder,and the maximum value is 419.8 K,which is located at the bridge passage of fourth cylinder two intake ports.When 2.0%volume fraction nanofluid is used as coolant,the distribution trend of temperature is same,but the maximum temperature of cylinder head and cooling water jacket decreased by 5.8 K and 4.4 K respectively.（5）Observing the flow field,it is found that the velocity distribution of both coolant is basically same,the maximum value is about 6.8 m/s,the average velocity basically meets the design requirements.The nanoparticles can enhance the disturbance degree of the base fluid and increase the flow velocity in the local areas,the streamline in bridge passages of third and fourth cylinders is sparse,and the flow velocity is lower than 0.5 m/s.Velocity of lateral and longitudinal cross-section shows that there are some phenomena such as low velocity and flow vortices in local positions.Due to the low flow velocity at bridge passage areas,the bubble volume fraction is larger,and the maximum value is 18%when the pure water is coolant.When the nanofluid is used as coolant,the volume fraction of bubble decreases,but the distribution range is wider,which is because the nanoparticles reduce the bubble separate diameter,and the bubbles are easier to annihilate in fluid to reduce the bubble volume fraction.Micro-grooves formed by the contact between nanoparticles and heating surface increase the vaporization core to make the bubble distribution range wider.