Study On Operational And Thermal Performance And Pipeline Configuration Improvement Of Closed Loop Pulsating Heat Pipe

Increasing power levels and miniaturization of electronic devices have made thermal management of the devices become an important issue. Pulsating heat pipe, as a new type heat transfer device with high heat transfer performance, has attracted deep concern of universities, research institutes and companies. Pulsating heat pipe has simple pipeline configuration while has strong thermo-hydro-dynamic coupling governing its operation state and thermal performance. The research on pulsating heat pipe is at primary stage, revealing its operation mechanism needs further more experimental investigations and theoretical analysis.Based on the analysis of effect factors on operational and thermal performance of pulsating heat pipe .The research objects of this dissertation are determined as follows: understanding the start-up regularity of closed loop pulsating heat pipe (CLPHP); further understanding operational and thermal performance of CLPHP; enhancing thermal performance by making improvement in pipeline configuration of CLPHP. The main contents are listed as follows:(1) Investigations and research carried on the start-up regularity of CLPHP. Bubble is the primary driving force for startup of CLPHP. When filling CLPHP with working fluid, the difference between saturation pressure of working fluid and absolute pressure in vacuumed CLPHP affects heatinput for bubble conformation. Directional flow of working fluid is mainly maintained by Gravity and pressure difference between condenser and evaporator in operational states. After startup, flow patterns dominate heat transfer mechanism, annular flow mainly exists in CLPHP at low filling ratio, higher thermal performance can be got when stable annular flow is kept in CLPHP.(2) Confirmation of heat-transfer limit in CLPHP. Dry-out pheanomena appears easily at low filling ratio or with high heatinput strength. The result strongly indicates that CLPHP has heat-transfer limit. Three characteristics reflect CLPHP reaches its heat-transfer limit: firstly, the annular flow in CLPHP is unstable and dry-out appears with enhancing of heatinput; secondly, decrease of temperature difference between inlet and out let of cooling water in condenser jacket indicates heat-transfer deterioration, the temperature in evaporator goes up sharply; thirdly, thermal resistance increases after dry-out appearance.(3) Proposal of pulsating heat pipe with improved pipeline configuration and investigations on its thermal performance. Local pressure drop of U turn affects both start-up and operation of the CLPHP. Flow pattern of working fluid at U turn is unstable annular flow, with heatinput increase, dry-out appears easily and reaches heat transfer limit. Improved pipeline configuration with parallel channels can form several loops which greatly reduce local pressure drop for working fluid. It also decreases thermal driven temperature difference between evaporator and condenser of the pulsating heat pipe, and thermal resistance of the pulsating heat pipe decreases. Furthermore, sufficient liquid working fluid can return from condenser to evaporator and dry-out appears hardly. It elevates heat transfer limit to some extent. When compared with typical multi-turn CLPHP, the improved pulsating heat pipe has better thermal performance.Experimental data about CLPHP has been accumulated and a new type CLPHP with better performance is proposed for engineering application.