Development And Heat Transfer Characteristics Study Of Concentric Annular High Temperature Heat Pipe

High temperature heat transfer problems widely exist in high temperature thermal protection,high temperature measurement calibration,industrial cooling and many other technical fields.High temperature heat pipe(HTHP),based on the heat transfer mechanism of the liquid metal phase-change circulating inside,has many high temperature heat transfer characteristics,such as good heat transfer ability and uniform temperature distribution,etc.These advantages make HTHP has a broad application prospect in the field of these high temperature heat transfer technology.However,in order to achieve a wider practical application of HTHP,it is still necessary to further optimize its structure,improve its comprehensive heat transfer performance,and solve the problems of anti-gravity,structure-performance relationship and design criterion,as well as the evaluation of temperature uniformity.Based on the concentric annular heat pipe structure which has a good liquid pumping capacity,with the combination of the internal two-phase flow mechanism and the experimental results,this paper systematically studied the main approaches to improve the heat transfer performance of the concentric annular high temperature heat pipe(CAHTHP).Meanwhile,the structure-performance relationship between the CAHTHP structure and its heat transfer performance was explored,the variation law of its temperature uniformity in spatial scales was revealed,and the design criterion of the high performance CAHTHP and the evaluation method of the temperature uniformity were established.The main research contents and conclusions of this paper are as follows:CAHTHPs with good temperature uniformity and high heat transfer performance were designed and developed.The effects of heat pipe structure,optimum liquid-filling rate range,composite mode of the capillary screens,length-diameter ratios and working angles on the CAHTHP heat transfer performance were systematically studied,and a design scheme for improving the heat transfer performance and the temperature uniformity of the CAHTHP was proposed.The results indicate that:the concentric annular heat pipe structure can significantly increase the heat transfer area and the liquid pumping capacity of the heat pipe,and strengthen its heat transfer capacity;the liquid sodium was used as the working medium,and the optimal liquid-filling rate of the CAHTHP is 15%;reducing the length-diameter ratio can significantly improve the heat transfer limit and the temperature uniformity of the CAHTHP,and the CAHTHP has a best heat transfer performance in the length-diameter ratio of 3,which the omnidirectional thermal resistance ranges is 0.006?0.015 ?/W,present only 1/3 of the conventional structure high temperature heat pipe;the capillary wick composite mode of dense-sparse-dense sequence can simultaneously take into account the advantages of large capillary force and high permeability,which can improve the heat transfer limit of the CAHTHP from 1297 W to 1896 W,and enhance the temperature uniformity at the mean while.The extreme heat transfer performance in the anti-gravity working condition(-90°)of the CAHTHP was systematically studied,the temperature variation regularity and the internal vapor flow mechanism of the CAHTHP in the frozen startup process were revealed.The results indicate that:the CAHTHP could be successfully started up under the anti-gravity working condition;the frozen start-up time was 660 s,which did related to the heat capacity and the heat dissipation power,and did unrelated to the working inclination of the CAHTHP;the largest anti-gravity working height of the CAHTHP could reached 300 mm,the corresponding thermal resistance as low as 0.021 ?/W,the effective thermal conductivity was more than 10000 W/m·K,which is the optimal level in the current literature reports.The quantitative description method of the relationship between structure and heat transfer performance of the high temperature heat pipe was proposed,the structure-performance relationship and the design criterion of the CAHTHP were obtained.The heat transfer analysis models were established for the condensing liquid film,the evaporation section and the capillary wick section of the CAHTHP respectively,and this three-zone heat transfer model was set up through the overall mass and energy conservation of the heat pipe,thus the implicit relationship between the structure parameters and the heat transfer performance was established,and the theoretical solution of the structure-performance relationship was solved by programming.By a dimensionless method,the paper obtained a non-dimensional number ? that can directly reflects the temperature uniformity of the CAHTHP,and a non-dimensional number SH that contains the structure characteristics and the heat transfer performance.Through a data fitting method,the ?-SH functional relation was obtained,thereby manifesting the structure-performance relationship of the CAHTHP.Through a serials of constant temperature heating experiments,the structure-performance relationship of the CAHTHP was experimentally verified,the results agreed well with the theoretical solution.According to the structure-performance relationship of the CAHTHP,the feasible methods of improving the CAHTHP's temperature uniformity and heat transfer performance were put forward from the aspects of working condition design and structure design of the heat pipe,thus the design criterion of the CAHTHP was formed.An evaluation method for generalized temperature uniformity was established which used the mean temperature deviation ?t and the spatial temperature uniformity f as evaluation indexes,the variation law of the temperature uniformity in spatial scales of the CAHTHP was studied.Based on the mathematical model of the two-phase flow inside the heat pipe,and taken ?.was less than 50‰ as the evaluation index,the temperature distribution inside the heat pipe under different working conditions was simulated and experimentally measured,and the spatial temperature uniformity performance of the CAHTHP was further studied.The results indicate that:the main method to improve the spatial temperature uniformity of CAHTHP is consistent with the design criterion;the maximum temperature uniform space in the heat pipe is mainly located in the condensation section of the heat pipe,and the average temperature of the temperature uniform space is close to the wall temperature of the condensation section;in the experimental range of this paper,the maximum spatial temperature uniformity ratio of the CAHTHP could reached 85.71%.