18 research outputs found
Effect of non-condensable gas on the startup of a loop heat pipe
It is essential to address the startup issues prior to the wide application of loop heat pipes (LHPs) in both space and terrestrial surroundings. As non-condensable gas (NCG) is an important factor affecting the startup behavior, its effects on the startup performance of an ammonia-stainless steel LHP with and without preconditioning were experimentally investigated in this work. Nitrogen with controlled amounts was used to simulate the NCG, and the temperature overshoot, liquid superheat and startup time were employed as the evaluation criteria. Four situations relating to initial liquid/vapor distribution in the evaporator were examined: (1) both evaporator core and vapor grooves are filled with liquid, (2) vapor exists in vapor grooves and the evaporator core is filled by liquid, (3) vapor grooves are filled by liquid and vapor exists in the evaporator core, and (4) vapor exists in both evaporator core and vapor grooves. Experimental results showed that with NCG presence in the LHP, the startup could only proceed in situation 1 with preconditioning, while it could proceed in situations 1, 3 or 4 without preconditioning. For the startup in situation 1, a larger NCG inventory led to much degraded startup performance, and a higher startup heat load could benefit the startup. For the startup in situation 3, the most difficult startup situation, NCG resulted in a very high temperature overshoot, which may even exceed the maximum allowable value. For the startup in situation 4, the existence of NCG in the vapor grooves could facilitate the evaporation there, leading to a very desirable startup
Steady-state modeling and analysis of a loop heat pipe under gravity-assisted operation
Loop heat pipes (LHPs) are efficient two-phase heat transfer devices that have found many space and terrestrial applications. This work addresses our insufficient understanding of LHP operation under gravity-assisted attitude, i.e. the condenser is located higher than the evaporator. A steady-state mathematical model of a LHP under gravity-assisted operation was established based on two driving modes: gravity driven mode and capillarity-gravity co-driven mode, determined by a defined transition heat load. The model was validated by the experimental results, and was employed to predict the operating characteristics of a LHP under the gravity-assisted attitude. Comparing to LHPs operating under horizontal or antigravity attitudes, some distinctive features have been identified, which include: i) the total mass flowrate in the loop shows a unique V-shape with the increase of applied heat load; ii) the steady-state operating temperature is much lower under the gravity driven mode, and is in similar values under capillarity-gravity co-driven mode and iii) the thermal conductance of the LHP increases with increasing positive elevation especially in the variable conductance zone. Such results contribute greatly to the understanding of the complicated operating principle and characteristics of LHPs especially for terrestrial applications
Experimental study of curvature effects on jet impingement heat transfer on concave surfaces
Experimental study of the local and average heat transfer characteristics of a single round jet impinging on the concave surfaces was conducted in this work to gain in-depth knowledge of the curvature effects. The experiments were conducted by employing a piccolo tube with one single jet hole over a wide range of parameters: jet Reynolds number from 27,000 to 130,000, relative nozzle to surface distance from 3.3 to 30, and relative surface curvature from 0.005 to 0.030. Experimental results indicate that the surface curvature has opposite effects on heat transfer characteristics. On one hand, an increase of relative nozzle to surface distance (increasing jet diameter in fact) enhances the average heat transfer around the surface for the same curved surface. On the other hand, the average Nusselt number decreases as relative nozzle to surface distance increases for a fixed jet diameter. Finally, experimental data-based correlations of the average Nusselt number over the curved surface were obtained with consideration of surface curvature effect. This work contributes to a better understanding of the curvature effects on heat transfer of a round jet impingement on concave surfaces, which is of high importance to the design of the aircraft anti-icing system
Experimental study on a dual compensation chamber loop heat pipe with dual bayonet tubes
Dual compensation chamber loop heat pipe (DCCLHP) holds great application potential in the future aircraft thermal management. In this work, a DCCLHP with dual bayonet tubes was first proposed and fabricated, aiming to improve its startup performance especially at small heat loads in the terrestrial surroundings. Extensive experimental validation was conducted at three typical attitudes of the evaporator/CCs, i.e., the vertical attitude, 45° tilt angle and the horizontal attitude, mainly focusing on its startup characteristics and heat transport capability. According to the experimental results, the DCCLHP with dual bayonet tubes can successfully realize the startup at small heat loads in whatever attitudes of the evaporator/CCs in the ground condition, and reach a heat transport limit greater than 400 W over a distance of 2.0 m. No obvious operating instability was observed in the DCCLHP operation. In addition, a new flow mechanism was observed in the experiment, i.e., a local natural circulation of the working fluid driven by gravity occurred in the loop composed of the evaporator, the CCs, the bayonet tubes, and the branches of the liquid line. This local circulation of working fluid was identified to appear only when the evaporator/CCs were at a certain tilt angle and the heat loads were relatively small
Experimental study on pool boiling in a porous artery structure
In this work, a porous artery structure is proposed to enhance the critical heat flux (CHF) of pool boiling based on the concept of “phase separation and modulation” and extensive experimental studies have been carried out for validation. In the experiment, multiple rectangular arteries were machined directly into the top surface of a copper rod to provide individual flow paths for vapor escaping. The arteries were covered by a microporous copper plate where capillary forces can be developed at the liquid/vapor interface to prevent the vapor from penetrating the porous structure and realize strong liquid suction simultaneously. The pool wall was made of transparent quartz glass to enable a visualization study where the liquid/vapor distribution and movement can be observed directly. Favorable results have been reached as expected, and a maximum heat flux up to 805 W/cm2 was achieved with no indication of any dry-out, which successfully validated this new concept. In addition, the effects of the diameter and thickness of the porous copper plate, and the connection method between the porous copper plate and copper fin on the pool boiling heat transfer in the porous artery structure were investigated, and the inherent physical mechanisms were analyzed and discussed
Effect of evaporator/condenser elevations on a loop heat pipe with non-condensable gas
The coupling effect of evaporator/condenser elevations and non-condensable gas (NCG) on the performance of a loop heat pipe (LHP) operating in gravitational field was investigated experimentally. Ammonia and nitrogen were selected as the working fluid of LHP and the simulated gas of NCG, respectively. The experiments were conducted at three kinds of evaporator/condenser elevations, namely zero elevation, adverse elevation and positive elevation. Experimental results show that NCG will cause an increase in operating temperature, but the trends varies at different evaporator/condenser elevation elevations. The temperature rises caused by NCG at the zero and adverse elevations are negatively correlated with heat load, and the maximum temperature increments are both at the minimum heat load of 15 W, but the influence of NCG is less at adverse elevation. On the other hand, at favorable elevation, the temperature rise exhibits different characteristics in different LHP operation modes, i.e., positively correlated with heat load in gravity driven mode and negatively in capillarity-gravity co-driven mode, and the transition heat load is 60 W. For an LHP that has already contained a certain amount of NCG, functioning at favorable elevation could eliminate the adverse effects of NCG on operating temperature and heat transfer performance to some extent. Furthermore, it is found that the presence of NCG and adverse elevation appears to inhibit the backflow during startup and improve startup stability. These results might have reference significance for the design and installation of the LHP for terrestrial applications
Development of cryogenic loop heat pipes: A review and comparative analysis
Loop heat pipes (LHPs) are highly efficient two-phase heat transfer devices with the ability to transport a large amount of heat over a long distance. Due to increasing demand of efficient cryocooling applications in both space and terrestrial surroundings, LHPs operating in cryogenic temperature range have been extensively investigated in recent years. This work provided a comprehensive review of the state-of-the-art of cryogenic loop heat pipes (CLHPs). Five different types of CLHPs were categorized, and a comparative analysis between CLHPs and ambient LHPs and among different types of CLHPs was conducted. More attention was paid to the supercritical startup of CLHPs, and the operation and performance characteristics of different types of CLHPs were compared in terms of system structure, supercritical startup, heat transport capacity and the effect of parasitic heat load. The parameters that affect the CLHP performance were analyzed, and the optimization strategy was proposed in order to progress their future development and engineering applications
Photothermal conversion efficiency of nanofluids: An experimental and numerical study
This work investigated experimentally the photothermal conversion efficiency (PTE) of gold nanofluids in a cylindrical tube under natural solar irradiation conditions, which was also compared with a developed 3-D numerical model. The PTE of gold nanofluids was found to be much higher than that of pure water, and increased non-linearly with the nanoparticle concentration, reaching 76.0% at a concentration of 5.8 ppm. Significant non-uniform temperature distribution was identified both experimentally and numerically, and a large uncertainty can be produced in the PTE calculation by using only one-point temperature measurement. A mathematical model was also established to calculate the solar absorption efficiency without knowing the temperature field within the nanofluids, which can be used to predict the theoretical PTE for nanofluids based on their optical properties only
Effect of evaporator tilt on the operating temperature of a loop heat pipe without a secondary wick
The effect of evaporator tilt on the operating temperature of a loop heat pipe (LHP) without a secondary wick under terrestrial surroundings was investigated both experimentally and theoretically in this work. The experiments were conducted with the evaporator placed at three different tilts: (a) the evaporator was horizontal with the compensation chamber (CC), (b) the evaporator was vertically below the CC, and (c) the evaporator was a little higher than the CC with a tilt angle of 1.80°. Experimental results show that the evaporator tilt has significant effect on the operating temperature of the LHP: the operating temperature at evaporator tilt (c) was much higher than those at two other evaporator tilts. Theoretical analysis based on a two-zoned evaporator wick, i.e. a subcooled zone and a saturated zone, is conducted to explain the experimental results, indicating that the cooling effect of the returning liquid on the vapor region in the CC or the evaporator core is crucial in determining the operating temperature of LHPs, which should be well considered in the LHP design and applications