Condensation Thermal Resistance Research Articles

Experimental investigation on the thermal performance of a novel loop heat pipe (LHP) with micro-channel structure

Loop heat pipe (LHP) is widely used to solve the problem of high heat flux in electronic chips due to its excellent heat transfer efficiency, good thermal equilibrium, and large heat transfer limit. To further improve the heat transfer efficiency of the LHP, a novel LHP was designed with a parallel arrangement of 13 vapor channels (2 mm × 2 mm) capillary wick structure. Experimental investigations were conducted to evaluate the start-up and steady-state performance of the LHP under both regular and random variable loads ranging from 30 W to 180 W. The findings indicate that the LHP exhibits rapid start-up and operates smoothly within the heat load range of 30 W–180 W, maintaining the heat source temperature below 75 °C, thereby meeting the temperature requirements for chip operation.Under a regular variable heat load, the LHP experiences minimal temperature fluctuations, but the system operated with good stability. Meanwhile, under random variable heat loads, the LHP demonstrates quick responsiveness within 5 s and reaches a stable state within 8 min. The temperature fluctuations remain within 2 °C, effectively fulfilling the temperature control demands of electronic devices. Furthermore, the evaporator thermal resistance, condenser thermal resistance, loop thermal resistance and system thermal resistance of the LHP gradually decrease with increasing of heat load. When reaching a heat load of 180 W, the evaporator thermal resistance (Re), condenser thermal resistance (Rc), loop thermal resistance (RLHP), and system thermal resistance (Rsys) reach their minimum, with values of 0.0168 °C/W, 0.121 °C/W,0.137 °C/W and 0.229 °C/W, respectively. In comparison with other LHPs, the evaporator temperature was only 59.56 °C with the highest heat load of 210 W (52.5 W/cm2), which expanded the application range of the LHP for electronic devices cooling.

Comparative Thermal Performances between Pumped Thermosyphon Loops with Different Condenser Configurations Using R245fa as Working Fluid

A pumped two-phase thermosyphon loop is broadly utilized to intensify the cooling duties of electronic chipsets/systems and the effectiveness for harvesting thermal energy. The configuration of a condenser not only affects the heat transfer in the condenser, but also has an effect on the saturation pressures during the boiling and condensation processes to alter the hydrothermal performance of a pumped thermosyphon loop. The influence of the condenser configuration on the hydrothermal performance of a pumped thermosyphon loop is rarely studied. The present study comparatively examined the thermal performances of two pumped thermosyphon loops with a conventional tube-fin condenser and the expansion-tank condenser. The thermodynamic cycles in pressure-temperature and pressure-enthalpy diagrams, Nusselt numbers of evaporator and condenser, thermal resistances and various performance indexes evaluated at constant pumping powers at the controlled through-flow Reynolds numbers, boiling numbers and condenser thermal resistances were measured. At the similar thermal loads, flow rates, and fluid entry temperatures of condenser, the operating pressure of the thermosyphon loop with expansion tank condenser was considerably reduced from that with tube fin condenser, leading to the lower saturation temperature for reducing the thermal resistance and the lesser pressure drop across the loop with a noticeable hydrothermal performance improvement. At the parametric conditions tested, the ratio of dimensionless overall thermal resistances between the loops with expansion tank condenser and tube fin condenser fell in the range of 0.81–0.99. When the loop performance was compared at a constant cooling airflow rate without considering the more pumping power consumption for the loop with tube fin condenser, the ranges of thermal resistance for the loops with expansion tank condenser and tube fin condenser were 0.13–0.21 (KW−1) and 0.15–0.23 (KW−1). The merit indices evaluating the comparative hydrothermal performances of evaporator, condenser and loop between the two looped thermosyphons highlighted the significance of condenser design and affirmed the performance improvement by changing tube fin condenser into expansion tank condenser. The empirical correlations of evaporator Nusselt number, condenser Nusselt number, and overall thermal resistance using Reynolds number, boiling number, and condenser thermal resistance as the controlling parameters were generated for relevant applications.

Experimental study on thermal performance of ethanol loop heat pipe with flat evaporator under gravity – assisted condition

Nowadays, extreme growing of telecommunication and information technology causes the significant changes in the electronics belonging to the high-performance computer, data centers that are miniaturizing in their size and increasing the heat power dissipation. As a result, it is important to study on new cooling methods rather than conventional air cooling to warrant electronic device operate durably and stably. Besides, saving electricity power consumed by cooling systems is another considerable concern. Loop heat pipe, a passive heat transfer device which functions based on phase change processes and natural forces like gravity or capillary force, has been being one of potential solutions for the above challenges. In this paper, a loop heat pipe with flat evaporator was fabricated to investigate its cooling characteristics during start-up, stable operating period at different heat loads under gravity–assisted condition. This loop heat pipe was charged with ethanol whereas sintered stainless steel wick was the capillary structure. Time for successful start-up shortened from 13 minutes to 4 minutes as heat load increased from 30 W to 225 W. When heating power supplied to loop heat pipe’s evaporator was adjusted from 30 W to 400 W (14.8 W/cm2), the heating block surface’s temperature increased from 33oC to 133oC. This temperature could be maintained below 85oC, electronics limitation temperature in industry, if heat power released from the heating block is smaller than 220 W (8.14 W/cm2). Besides, the change of different types of thermal resistance such as total thermal resistance, evaporator and condenser thermal resistances with heating power is discussed detail in this paper.

Experimental investigation on the thermal performance of air-cooled multi-port flat heat pipes

An experimental investigation is conducted to study the effect of the type of working fluid, liquid filling ratio (FR) and the cooling and heating conditions on the performance of air-cooled multi-port flat heat pipes (AMFHPs) designed for the thermal management of power battery. The results show that, under the same FR of 50%,the ammonia AMFHP owns the highest thermal performance, whose equivalent thermal conductivity is 2.21–3.30 times that of one filled with acetone and 1.05–1.31 times that of one using R134a as working substance. Evaporation thermal resistance of acetone AMFHP is significantly larger than those of the AMFHPs filled with ammonia and R134a, whereas their condensation thermal resistance values are comparative. Secondly, it is found that the equivalent thermal conductivity of the R134a AMFHP with 50% FR is 1.63–2.22 times that of one with 30% FR and 1.14–1.63 times that of one with 70% FR, respectively. Thirdly, result indicates that the effect of cooling air velocity on the equivalent thermal conductivity of AMFHP is insignificant, but the temperature of evaporation section is tightly connected to the cooling air velocity. There exists an economic air velocity for each AMFHP, the appropriate air velocities of the AMFHPs filled with R134a, acetone and ammonia at FR=50% are 3.5 m/s, 2.5 m/s and 3.5 m/s, respectively. In addition, analysis of heat transfer reveals that, the intensity of heat transfer mode of the AMFHPs in liquid pool ranges from R134a, ammonia and acetone, under the same working condition and filling ratio. The most direct conclusion is that the R134a AMFHP with appropriate FR is suitable to meet the thermal management requirements of lithium ion batteries for electric vehicles.

Numerical modeling of vapor condensation over a wide range of non-condensable gas concentrations

This paper presents a numerical model to study the process of vapor condensation on surfaces characterized by film-wise condensation with the presence of Non-condensable gases (NCG). State variables in both the condensate film and the diffusion layer were solved separately and the condensation interface was used to couple the two solutions. The solution of the condensate film was obtained using well-established solutions of laminar film condensation of pure vapor. In contrast to other models surveyed, this work provides a inexpensive and accurate predictions of heat and mass transfer characteristics. We validated the work against two classical condensation problems. The model was first validated against empirical correlations and experimental work, resulting in a very good agreement. We then assessed the applicability of ignoring the condensate film effect, as performed in previous models, on the condensation processes by observing the thermal resistances of both the condensate film and diffusion layer. Results indicated that for the studied cases of NCG mass fractions above 20%, the condensate thermal resistance was at least an order of magnitude lower than that of the diffusion layer. However, the two thermal resistances seem to approach each other as NCG mass fraction becomes smaller. On another front, we observed that models that ignore the condensate film thermal resistance underestimate the interfacial temperature albeit accurately predicting the overall heat transfer rate. To simulate even lower NCG mass fractions, we validated our model to the classical analytical work of Sparrow and co-workers. Results showed a striking agreement between the two solutions at different NCG mass fractions (0.5%–10%) and subcooling degrees (5∘F–40∘F). Finally, we found a good agreement between results of our model and the heat/mass transfer analogy. The heat/mass transfer analogy is a semi-empirical method therefore, is limited to the existing correlations and their uncertainties. On the other hand, our model does not use any empiricism and relies on the available solutions of laminar condensate film of pure vapor in predicting the liquid side heat transfer coefficient.

Thermal performances study on a façade-built-in two-phase thermosyphon loop for passive thermo-activated building system

Pipe-embedded building envelopes, as a burgeoning active insulation technique that has its pipe circuits inside the building envelope, are of particular interest to architects and engineers due to their excellent performances and invisibility characteristic. However, performances of this pump-driven technique highly rely on long-term electricity consumption, resulting in degradation of system efficiency in building load reduction and supplementary heating. In the present study, a facade-built-in two-phase thermosyphon loop (TPTL) for passive thermo-activated building system (i.e. TPTL-embedded building envelope) is proposed to turn the pump-driven heat injection process of active insulation technique into non-pump-driven. Experiments on a prototype are conducted to explore operational characteristics of the TPTL whose condenser is operated in the thermal boundary of Dirichlet and dynamic thermal response characteristics of the composite building envelope after coupling TPTL at different heat loads and filling ratios. Results show that the filling ratio has a critical impact on thermal resistances and startup behavior of TPTL, and the optimal filling ratio (around 116%) is not a fixed value at different operating conditions. Besides, condenser thermal resistance occupies the largest proportion among the system thermal resistance due to the heat accumulation phenomenon, therefore, the interlayer material also has a great impact on heat injection and release of TPTL-embedded building envelope. In addition, reverse startup and operation under specific test condition are observed and discussed accordingly. Overall, these results and conclusions are useful for the system design, operation control, and further investigations on the façade-built-in TPTL and the TPTL-embedded building envelope.

Experimental study of condensation heat transfer on a horizontal aluminum tube with superhydrophobic characteristic

In this study, we conducted a condensation heat transfer experiment for an observation of the condensation heat transfer performance from a superhydrophobic surface fabricated with the Self Assembled Monolayer (SAM) method. For the experiment of condensation heat transfer, we designed an experimental facility that could compare a superhydrophobic tube and a bare tube under the same conditions. The experiment was conducted according to a condition number defined by the coolant Reynolds number and the saturation pressure, and two repeat tests were carried out to analyze the attached condensation. Each condensation phenomenon was observed by a camera, and the droplet detaching frequency was measured in units of [drops/min]. As a result, the dropwise condensation was observed under the condition #1 and #2 in the superhydrophobic tube, and the performance was improved by about 105% over the bare tube with regard to the average overall heat transfer coefficient. Under the conditions #3 and #4, the flooded condensation was observed, and the performance improved by about 16%. By contrast, the attached condensation with a lower detaching frequency than the flooded condensation was observed under the condition #5, and a performance degradation of about 20% was observed. In addition, the condensation performance was analyzed with a ratio of the condensation thermal resistance to a total thermal resistance. As a result of the condensation thermal resistance, the ratio decreased greatly than that of the bare tube when the dropwise condensation occurred, and it was slightly decreased in the condition of the flooded condensation, and it increased in attached condensation. In the repeat tests, after passing the attached condensation, we observed that the performance of the condition #2 was further decreased compared with the situation after passing only the flooded condensation. From the result, attached and flooded condensation seemed to be related to the droplet pinning depth, and we predicted that the droplet pinning depth of attached condensation would be deeper than that of flooded condensation. In conclusion, attached condensation more seriously affects the condensation performance degradation than flooded condensation. Therefore, it is necessary to understand the condition of attached condensation and to prevent attached condensation phenomenon.

Thermal performance of reciprocating two-phase thermosyphon with nozzle

Flow images and thermal performances of the two-phase reciprocating thermosyphons (RT) with an internal nozzle at 50% volumetric filling ratio (FR) were measured. Two sets of flow snapshots and thermal performance data detected from two similar RTs with and without nozzle were compared. Above the critical reciprocating frequencies (fcr), the circulation routes of working fluid in the RTs with and without nozzle were reversed to cause disparities in thermal performances. At the reciprocating frequencies (f) of 1.75, 1.83, 1.92 and 2 Hz with four different heating and cooling duties for each fixed f, the time-mean local and regionally averaged Nusselt numbers (Nu) along the evaporator/condenser centerlines of the RTs with and without nozzle were dominated by reciprocation number (Reci) but subject to the coupled effects attributed from the heating and cooling duties indexed by boiling number (Bo) and dimensionless condenser thermal resistance (Rth,con). Local Nu were raised by increasing Reci, Bo and/or Rth,con for both RTs with and without nozzle. Due to the inversed liquid film circulation and jet impingement, the overall thermal resistances (Rth) of present RT with nozzle were reduced by 40.06–59.96% from those of RT without nozzle. To add engineering applications, evaporator Nu correlations were devised for present RT with nozzle.

Identification of heat exchange process in the evaporators of absorption refrigerating units under conditions of uncertainty

Analysis of the evaporator of the absorption refrigerating secondary condensation unit of ammonia production was performed. The need to minimize the temperature mode of operation of the evaporator, which provides for energy efficiency in ammonia production by decreasing natural gas consumption was substantiated. Statistical processing of experimental data with assessment of the quality of obtained information was performed. Identification of the heat exchange process of the evaporator was performed and the equation for numerical evaluation of uncertainty of coefficients of thermal efficiency, heat transfer and condensation thermal resistance was obtained. According to the results of statistical modeling of the primary condensation unit, the equation for numerical evaluation of uncertainty of ammonia concentration in circulating gas at the evaporator inlet. The authors proved the possibility of its application for identification and mathematical modeling of heat exchange process at existing relationship of ammonia concentration in the circulating gas at the evaporator inlet and condensation thermal resistance. Algorithmic support of numerical evaluation of uncertainties and calculation of target indicators of the evaporator’s efficiency was created. The technical structure of the automated system for identification and timely obtaining of the mathematical model of the evaporator, which is adapted to the existing information system of the industrial ammonia synthesis unit, was developed.

System analysis of the secondary condensation unit in the context of improving energy efficiency of ammonia production

We established the mismatch between real and design coefficients of heat transfer, which is predetermined by the underestimation of condensation thermal resistance. The processes of heat exchange in a condensation column are identified and the equations for determining the coefficients of heat exchange, heat transfer and the concentration of ammonia in the circulation gas at the outlet of the column are derived. By applying the method of mathematical modeling, we defined necessary conditions for temperature distribution to exclude from the circuit of the unit for synthesis a turbo-compressor refrigeration unit with electric drive of capacity up to 4000 kW·h and reduction in the cooling temperature of circulation gas from 0 °C to – 5 °C at maximum thermal load with circulation gas on the complex of secondary condensation. The hardware-technological design is developed for the stage of secondary condensation based only on the absorption and steam-ejector refrigeration systems that utilize the heat of material flows of low (above 100 °C) and super-low (to 90 °C) potentials. We defined efficiency indicators for the reduction of specific energy consumption by electricity and natural gas, which are 60 kW·h/t NH 3 and 1.2 m 3 /t NH 3 .

Thermal performance of ultra-thin flattened heat pipes

This study investigates the thermal performance of composite ultra-thin heat pipes (UTHPs). UTHPs are fabricated by flattening cylindrical heat pipes with outer diameter of 2 mm. The thickness and width were 0.8 mm and 2.7 mm, respectively. The composite wick structure is made of sintered copper foam-mesh wick (CFMW). CFMW combines the good heat transfer performance of copper foam and the high mechanical strength of mesh. The manufacturing process of UTHP was studied and the thermal performance of UTHP samples were investigated experimentally. The results indicate that the optimum filling ratio of UTHPs is 100% and the maximum temperature difference is 3.7 °C under the maximum heat transport capacity of 5 W. The thermal resistances of UTHPs increase gradually with the heat power before drying out. Too low or too high filling ratios will reduce the heat transfer efficiency of UTHPs by increasing the thermal resistances. With the optimum filling ratio of 100%, the evaporation thermal resistance of UTHP is found to be 0.29 K/W and the condensation thermal resistance is 0.45 K/W at the heat load of 5 W.

Thermal performance of two-phase thermosyphon loop in rotating thin pad

Thermal performances of a sub-atmospheric Rotating Two-Phase Loop Thermosyphon Pad (RTPLTP) of 1 mm channel height were experimentally studied. Flow structures of vapor-liquid circulations, boiling/condensation heat transfer rates and thermal resistances (Rth) for the RTPLTP were detected at rotor speeds of 0, 100, 300 and 500 rev/min. With each rotor speed tested, experiments were performed at four ascending boiling numbers (Bo) with four descending non-dimensional condenser thermal resistances (Rth,con) at each Bo. Parametric test conditions specified by Bo, Rth,con and relative centrifugal acceleration (Ω) were 0.16–0.82, 0.07–0.3 and 0–28.2, respectively. Acting by the rotational force effects, the churn flow boiling structures in present thin static TPLTP initially transited into intermittent boiling flow structures with the temporal drifts of vapor bubbles along the vortical flow pathways in the rotating pad at Ω = 1.1. Further increasing Ω, the boiling flow structure yielded to the swaying and then the stable continuous tiny-bubble streams centered at the nucleate sites on the rotating boiling surface. Both averaged Nusselt numbers over evaporator (Nueva) and condenser (Nucon) of present RTPLTP increased as Ω increased. While Nucon kept increasing as Bo increased at all Ω tested, increases of Bo initially reduced Nueva at low Ω but turned to elevate Nueva at high Ω due to the Ω-induced variations in boiling flow structures. With the combined Bo and Ω effects on Nueva and Nucon, the total thermal resistances (Rth) of present thin RTPLTP decreased as Bo and/or Ω increased; but were increased by raising Rth,con. Empirical correlations evaluating Nueva, Nucon and Rth with Bo, Ω and Rth,con as the determining variables were devised to assist the relevant applications.

Thermal performance of ultra-thin flattened heat pipes with composite wick structure

This study proposes three composite wick structures (copper power or mesh sintered on grooved tube), namely, single arch-shaped sintered–grooved wick (SSGW), bilateral arch-shaped sintered–grooved wick (BSGW), and mesh–grooved wick (MGW), to improve the thermal performance of ultra-thin heat pipes (UTHPs). Phase-change flattening technology is employed to fabricate UTHPs. The morphologies of the wick structures after flattening are observed. An experimental apparatus is setup to investigate the thermal performance of UTHP samples under incremental heat loads. The heat transfer limits of UTHP are theoretically and experimentally analyzed. Capillary limit is found to be the main heat transfer limit, and the theoretical values of the samples with SSGW and BSGW are in good agreement with the experimental results. Results indicate that the maximum heat transport capacities are 12W, 13W and 14W, under the corresponding optimum filling ratios of 70%, 70%, and 80%, for the SSGW, BSGW and MGW UTHPs, respectively. Evaporation and condensation thermal resistances of UTHP samples increase with the increase in the filling ratio before the occurrence of dry-out. UTHPs with SSGW have the least evaporation thermal resistance whereas UTHPs with MGW have the least condensation thermal resistance.

Effects of graphene oxide nanofluids on heat pipe performance and capillary limits

In the present study, thermal performances of water-filled and 0.01 and 0.03 vol% graphene oxide (GO)/water nanofluids-filled heat pipes with a screen mesh wick were studied in order to investigate the effects of nanofluids on the heat pipe operation. The wall temperatures of the GO/water nanofluids-filled heat pipes were found to be lower than those of the water heat pipe. Also, the heat pipes charged with GO/water nanofluids showed lower evaporator thermal resistances by about 25% compared with the water-filled heat pipe, although the condenser thermal resistances were similar in both cases. The 0.01 vol% GO/water nanofluid-filled heat pipe showed better boiling heat transfer than 0.03 vol% GO/water nanofluid because of the different structures of the deposited nanoparticle layers on the wicks. The capillary limit of the heat pipes containing GO/water nanofluids was higher than for the water heat pipe, because the nanoparticles-coated layer modified the effective capillary radius and meniscus, resulting in an increase of the maximum liquid flow rate through the wick structure. The SEM images and wetting properties for wicks of the heat pipes with nanoparticles-coated layers support the thermal performance characteristics of the heat pipes obtained in the study.

Comparison of thermal performances of water-filled, SiC nanofluid-filled and SiC nanoparticles-coated heat pipes

In the present study, thermal performances of water-filled and 0.01 and 0.1vol% SiC/water nanofluids-filled heat pipes with a screen mesh wick and water-filled heat pipe with a SiC nanoparticles-coated screen mesh wick were compared in order to investigate the effects of nanoparticles depositions on inner surface structures of heat pipes. The wall temperatures of the SiC nanoparticles-coated heat pipe were found to be higher than those of an uncoated heat pipe while the thermal performance for the heat pipe using a SiC/water nanofluid was not enhanced compared to the heat pipe using water as a working fluid. Also, the heat pipes containing SiC/water nanofluids and SiC-coated wick showed slight increases in evaporator thermal resistances, but minute changes in condenser thermal resistances compared to the water-filled heat pipe. Moreover, the overall thermal resistances of the heat pipes with the SiC/water nanofluids and SiC-coated wick were similar with those of the heat pipe charged with water. In terms of heat transfer performance, the boiling heat transfer of the evaporator zone is explained by changes of the number of activated nucleation sites due to nanoparticle deposition on the wick structure, whereas heat transfer characteristics of the adiabatic and condenser zones are attributed to the liquid film layer which is formed on the wick structure by capillary wicking and is considerable as additional thermal resistance. The SEM images for wicks of heat pipes with nanoparticles-coated layers support the thermal performance characteristics of three types of heat pipes investigated in the study.

Thermal performance of thin loop-type vapor chamber

This experimental study examines the thermal performances of a newly devised thin loop-type vapor chamber (VC) of 0.5mm interior height at the controlled evaporator heat fluxes (q̇) and condenser thermal resistances (Rth,con) with vertical and horizontal orientations. The capillary layer over the entire heated VC endwall is constructed by the 0.05mm deep parallel/staggered square grooves with a copper woven wire mesh sintered above the staggered grooves on the evaporator endwall. The vapor–liquid flow images and their responsive overall thermal resistances (Rth) and effective spreading thermal conductivities (keff) are simultaneously detected with the objective to study the effects of q̇, Rth,con and VC orientation on the vapor–liquid two-phase flow phenomena and the associated heat transfer performances. At various test conditions, the responsive Rth and keff results to the interfacial flow structures, the comparative Rth and keff properties as well as the relative thermal performance improvements from the copper plate for the test VC are examined.

Thermal performance of sub-atmospheric two-phase loop thermosyphon at pitching and rolling conditions

With cooling applications to shipping machineries, thermal performances of static and swinging two-phase loop thermosyphons (TPLTs) operated at sub-atmospheric conditions are comparatively examined. The influences of pitching and/or rolling oscillations on the boiling flow structures and associated heat transfer characteristics, thermodynamic cycles and the constituent thermal resistances along the TPLT flow pathway are investigated. Flow visualization and thermal performance tests are individually performed at the identical test conditions controlled by boiling heater power (Q), condenser thermal resistance (Rth,con) and pitching/rolling frequencies to detect the boiling flow structures, boiling heat transfer rates, phase-change pressures in evaporator/condenser as well as the loop-wise compositions of the constituent thermal resistances. With present swinging TPLTs, the vapor bubbles in liquid pool yield to the disk-shape with the swaying bubble string centered at the nucleate site. Instead of the rigorous vapor bubble shooting in static evaporator channel at continuous boiling flow regime, the coherent vapor bubble cluster develops in the vapor dome of present swinging evaporator channel. Due to the swinging force effects on the boiling flow structures, the regionally averaged boiling heat transfer rates are initially reduced from the static references and followed by the subsequent increase as the swinging forces increase. The differential swinging force impacts at single pitching, single rolling and compound pitching and rolling test conditions on the boiling flow structures and heat transfer rates as well as the constituent and total thermal resistances are illustrated using a set of comparative results collected from the swinging and static TPLTs. The thermal resistances caused by boiling instabilities for present swinging TPLTs are considerably raised from the static references, leading to the noticeably elevations of cooling power consumptions. Two sets of empirical correlations, which permit the evaluations of boiling heat transfer rates and cooling power consumptions at various Q, Rth,con and pitching (Pi) or rolling (Ro) numbers are generated to assist the on-board applications.

Thermal performance of rotating two-phase thermosyphon disc

This experimental work examines the thermal performances of rotating two-phase thermosyphon disc (RTPTD) using centrifugal forces to drive vapor–liquid circulation at sub-atmospheric pressures with cooling applications to electrical rotor machineries. Boiling flow images, boiling/condensation heat transfer rates and the composition of total thermal resistances (Rth) for the RTPTD are examined at rotor speeds of 250, 500, 750 and 1000rev/min with four sets of boiling numbers (Bo) and condenser thermal resistances (Rth,con) at each rotor speed. Experimental conditions in terms of Bo, Rth,con and relative centrifugal acceleration (Ω) are in the respective ranges of 70–153, 0.065–0.25KW−1 and 4.3–70. While both boiling and condensation heat transfer rates are raised by increasing rotor speed, heat transfer properties over the evaporator and condenser of present RTPTD are respectively improved and impeded by raising Bo. Due to the combined Bo and Ω effects on each constituent thermal resistance of Rth, the total thermal resistance of the present RTPTD is reduced by increasing Bo and/or Ω. Empirical correlations determining the boiling/condensation heat transfer rates and the evaporator pressures as well as Rth that permit the evaluations of individual and interdependent effects of Ω, Bo and Rth,con are generated to assist engineering applications.

Thermal performance improvement with scale imprints over boiling surface of two-phase loop thermosyphon at sub-atmospheric conditions

Thermal performances of two-phase loop thermosyphons (TPLT) with smooth and scale-roughened boiling surfaces are compared by examining their boiling flow structures, boiling heat transfer properties with the associated instability phenomena, thermodynamic cycles, boiling flow maps and networks of thermal resistances along the flow pathway. With cooling applications to electronic chipsets which limit the range of operating temperatures for these TPLTs, the working fluid of treated water has to circulate at sub-atmospheric pressures. With each tested TPLT, the flow visualization and thermal performance tests are individually performed at the identical test conditions controlled by boiling heater power (Q) and condenser thermal resistance (Rth,con) to image the boiling flow structures and to measure the boiling heat transfer rates, phase-change pressures in evaporator and condenser and the loop-wise temperature distributions, respectively. The differential Q and Rth,con impacts on boiling flow structures and thermal performances between the TPLTs with smooth and scaled boiling surfaces are illustrated using a set of comparative results collected from these TPLTs. With the scaled boiling surface over evaporator at the filling ratio (FR) of 50%, the Q-driven transition of boiling flow structures still follows the smooth-walled TPLT route; but the increase of nucleation sites and the improved bubble departure from the scaled surface elevate the boiling heat transfer rates. Particularly, the segmentation of Taylor bubbles by the scale imprints considerably suppresses the oscillatory amplitudes of liquid level and temperatures in evaporator; and thereof enhances thermal stabilities to reduce the constituent thermal resistance triggered by boiling instabilities. Acting together by the suppression of boiling instabilities and the improved boiling heat transfer properties, the cooling powers required for the scaled TPLT are noticeably reduced from the smooth-walled TPLT counterparts at the conditions with similar overall thermal resistances. Heat transfer correlations determining the boiling heat transfer coefficients over pool-boiling, intermittent and vapor regions for the scale-roughened evaporator along with the empirical correlations that permit the evaluation of individual and interdependent Q and Rth,con effects on the overall thermal resistances and evaporator pressures of the scaled TPLT are generated to assist design activities.

Evaporation and condensation heat transfer in a heat pipe with a sintered-grooved composite wick

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.