| For water-cooled internal combustion engine, boiling heat transfer is an important heat transfer mode of cooling water-jacket in cylinder head. At present, with the increasing requirements of fuel economy and exhaust emission of vehicle, the requirements of cooling system are also rising. It is the trend of cooling system in future engine to achieve high-temperature cooling by means of the high heat transfer capacity of nucleate boiling in cooling water-jacket. The key to implement the technology of high-temperature cooling is precise control of boiling heat transfer state in cooling water-jacket. For the complex structure of cooling water-jacket in cylinder head, the heat loads and heat transfer conditions in different parts of the wall vary greatly, without effective control of boiling state, transition boiling, even film boiling may occur in local parts, becoming the security risks of engines. Therefore, in order to understand, predict and evaluate the boiling heat transfer conditions of cooling water-jacket in cylinder head more accurately, the boiling heat transfer under complex flow and heat transfer conditions in cooling water-jacket is studied deeply in this paper from the following aspects.1) To meet the requirements of UDS interface in CFD software for solving differential equations, void mass fraction was used to reflect the mass portion of bubbles in the fluid with co-existing vapor and liquid. A conservative void mass fraction equation was established through theoretical analysis and mathematical derivation. Then a new homogenous flow boiling heat transfer model was established with the flow boiling heat transfer flux calculated by the widely used test correlation recommended by Rohsenow and Incropera. By embedding the calculation module of boiling heat transfer model into CFD software using VC++ as computer language, it was realized to solve the mixed flow with single-phase flow equations and void mass fraction equation. After obtaining the void mass fraction, the void fraction distribution of the flow was estimated indirectly according to the relationship between void mass fraction and void fraction, which can be helpful to intuitively understand the boiling condition in flow from the perspective of two-phase flow.Numerical simulation on the boiling heat transfer process in the experimental cooling gallery was conducted using the homogenous flow boiling heat transfer model established in this paper, the velocity, temperature and void fraction distribution of the coolant in the experimental cooling gallery were obtained. In order to validate whether the new model is applicable, calculation results and experimental results about the heat flux of the wall at 75 different operating conditions were compared. The comparison shows that the trends of the calculations and measured results are the same along with the temperature change in the wall; the average relative error of the calculations and measured results is 3.5% and the relative errors are no more than 6% in most of the conditions apart from four conditions with relative error of more than 8%, validating that the model is feasible.2) In order to understand the heat load conditions and the boiling heat transfer conditions in cooling water-jacket of cylinder head, and to provide the validating basis for the liquid-solid three-dimensional numerical simulation of cooling water-jacket, the temperature both of the fire deck and interior of the cylinder head and the cooling water-jacket wall in 226-B diesel engine were all measured using hardness plug and copper-constantan thermocouple separately on the self-designed and built temperature measurement test stand for cooling system of engine, relative accurate test data of temperature both about the cylinder head and the cooling water-jacket wall were obtained.The experimental results show that the temperature distribution of cylinder head is in line with the regular pattern that the temperatures on fire deck are higher than the other parts, the temperaturs of exhaust pipe wall are higher than that of intake pipe wall, and the temperatures on cooling water-jacket wall are about 100℃. Under rated condition, the maximum temperature on the fire deck of the cylinder head is 298℃, which does not exceed the allowable limit, indicating that the temperatures of the whole cylinder head are in a safe range. It is known from the temperature distribution of the fire deck that the maximum temperature appears in the bridge zone, followed by the outside area of the bridge zone near the exhaust pipe, the measured results are in accord with the actual situation that the highest temperature appears in the bridge zone and the temperature of the exhaust pipe side is generally higher than the intake pipe side. The maximum temperature on the cooling water-jacket wall of cylinder head is 121℃, indicating that boiling occurs in local regions of the cooling water-jacket;3) In order to investigate the impact of boiling on the heat transfer in cooling water-jacket of cylinder head, fluid-solid coupling numerical simulation on the flow and heat transfer of cooling water-jacket in cylinder head of Duets 226-B diesel engine were conducted respectively using the single-phase flow convective heat transfer model in CFD software when not considering boiling and the homogenous flow boiling heat transfer model built in this paper when considering boiling.By comparing the calculation results of the two models with the experimental results, it is found that the average absolute error of the temperature on the fire deck of cylinder head obtained by the single-phase flow convective heat transfer model is 26.8K, the average relative error is 2.67%; the average absolute error of the calculated temperature 7mm far from the fire deck is 15.OK, the average relative error is 2.78%. The average absolute error of the temperature on the fire deck of cylinder head obtained by the homogenous flow boiling heat transfer model is 11.8K, the average relative error is 1.42%; the average absolute error of the calculated temperature 7mm far from the fire deck is 10.7K, the average relative error is 2.47%. From the data above, it is shown that the numerical results of the homogenous flow boiling heat transfer model are more close to the experimental results, indicating that considering boiling heat transfer is more in line with the actual situation of flow and heat transfer in cooling water-jacket of cylinder head.By comparing the calculation results of the two models with each other, it is found that in the case of considering boiling heat transfer, the heat transfer coefficient in some high temperature areas increase significantly, with about 2.5 times in the interface of injector and fire deck, about 2 times in the triangle area of the bridge zone and about 1.5 times in the peripheral of the exhaust pipe; however, the temperature on fire deck of cylinder head and most region of the cooling water-jacket wall decline generally, with 5K in the maximum temperature on the fire deck in cylinder head and 17K in the maximum temperature on the cooling water-jacket wall. The comparison result indicates that boiling plays an evident role in enhancing heat transfer of cooling water-jacket in cylinder head, which should not be overlooked in numerical calculation.4) Adding boiling heat transfer model is not only difficult but also not permitted in some CFD software, so a Corrected algorithm for the boiling heat transfer calculation in cooling water-jacket of cylinder head was proposed, based on the homogenous flow boiling heat transfer model built in this paper. For the Corrected algorithm, simple pure convection heat transfer process is simulated firstly by means of CFD software and the calculated heat flux and convection heat transfer coefficient are output, then the output heat flux and convection heat transfer coefficient are amended outside of the CFD software using the program prepared according to the correction formula, thus the integrated heat flux and integrated heat transfer coefficient with consideration of boiling heat transfer can be obtained.Using the Corrected algorithm, fluid-solid coupling numerical simulation on the boiling heat transfer of cooling water-jacket in cylinder head of 226-B diesel engine was conducted, the calculation results of the Corrected algorithm are compared with the experimental results and the calculation results of the homogenous flow boiling heat transfer model separately.Compared with the experimental results, the average absolute error of the temperature on the fire deck of cylinder head is 12.7K, the average relative error is 1.53%; the average absolute error of the calculated temperature 7mm far from the fire deck is 10.9K, the average relative error is 2.50%. Compared with the calculation error of the homogenous flow boiling heat transfer model, it can be found that the calculation errors of both methods are basically the same, indicating that the Corrected algorithm is accurate enough in simulating the boiling heat transfer phenomena of cooling water-jacket in cylinder head.Contrast with the calculation results of the homogenous flow boiling heat transfer model, the heat transfer coefficient distribution on the cooling water-jacket wall of cylinder head calculated by both methods are consistent, and the calculation results are almost the same with the average relative error of 3.5%; the temperature distribution on the fire deck and the cooling water-jacket wall calculated by both methods are also basically the same, with a difference of 0.5K in maximum temperature of the fire deck and 0.6K in maximum temperature of the cooling water-jacket wall.For further details, the temperatures at different positions of the wall in cylinder head calculated by both methods are compared, it is found that the temperature differences between the two methods are very small, no more than 1.8K basically; but the comparison of calculating time shows that the coupling convergence velocity of Corrected algorithm is faster and the calculating time is significantly shorter with time saving of about 40% per calculation, indicating that the Corrected algorithm is of great value to engineering applications.5) In order to describe accurately the state of boiling heat transfer of cooling water-jacket in cylinder head, the concept of average void fraction was proposed and was used as the symbol to reflect boiling heat transfer state. The reasonability of calculating the average void fraction in 5 mm sectional reference height was validated through a great deal of numerical simulation on the experimental cooling gallery.By studying the transition mechanism among various boiling heat transfer states deeply, it was pointed out that the method current widely used to evaluate the limit state of nucleate boiling using the coolant velocity of 0.5m/s as the criterion has flaw. In order to determine the boiling heat transfer state of cooling water-jacket in cylinder head, it was proposed based on the concept of the average void fraction that the critical value of average void fraction be used as the criterion for judging the limit state of nucleate boiling of cooling water-jacket in cylinder head and be used to evaluate whether the boiling heat transfer in cooling water-jacket of cylinder head exceed the limit state of nucleate boiling. By a large number of numerical calculation and analysis, the relationship between average void fraction and coolant velocity, pressure, and wall superheat were determined, and the critical values of the average void fraction in the limit state of nucleate boiling were obtained.According to the criterionαcr for judging the limit state of nucleate boiling, average void fraction a of 24880 elements near the wall of cooling water-jacket in cylinder head of 226-B diesel engine calculated by the homogenous flow boiling heat transfer model were checked. It was found through the comparison that the average void fraction in every element is less than its' critical value, indicating that the nucleate boiling heat transfer in cooling water-jacket of cylinder head is basically safe.
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