The effect of substrate conduction on the thermal performance of a flat plate pulsating heat pipe

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Pulsating Heat Pipes (PHPs) are proposed as one of promising cooling techniques of high heat flux devices. PHPs can be classified into two groups, tubular PHPs made from a capillary tube bent into many turns and Flat-Plate PHPs (FP-PHPs) which have channels engraved on a base plate. The biggest difference between two PHPs is the heat conduction through the solid wall of FP-PHP, while the conduction effect in tubular PHPs is negligible. Therefore, there are two main heat transfer paths in FP-PHPs, axial conduction through the solid wall and heat transfer by rapid self-sustained oscillation of working fluid. However, there also exists transverse conduction between neighboring channels through thin channel wall. The transverse conduction influences on the heat transfer by working fluid, since it reduces the thermally created oscillating motion of working fluid. By restricting the transverse conduction, the thermal performance of FP-PHPs was enhanced by more than 30% due to larger amplitude oscillation of working fluid. Moreover, early start-up and smaller fluctuation in evaporator temperature were obtained by confining the transverse conduction.cf201

Immunogenicity and Safety of Diphtheria-tetanus Vaccine in Adults

This study was conducted to evaluate the immunogenicity and safety of diphtheria-tetanus (Td) vaccine in adults over 40 yr old who had never received a diphtheria-tetanus-pertussis (DTP) vaccination. A total of 242 subject completed three-doses of Td vaccination and subsequent assays for immunogenicity. Before vaccination, 33.9% and 96.7% participants showed antibody levels of diphtheria and tetanus, respectively, which were below protective level (<0.1 U/mL). After the first dose of Td vaccine, 92.6% and 77.6% of subjects gained protective antibody concentrations (≥0.1 U/mL) for diphtheria and tetanus, with an increase to 99.6% and 100% after the third dose. Local and systemic adverse events occurred in 37.9% and 15.5% of the subjects. No serious adverse event requiring an unscheduled hospital visit occurred. In conclusion, three-doses of Td vaccination to unimmunized adults are safe and effective in inducing protective immunity against diphtheria and tetanus

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The need for innovative thermal control systems for space equipment such as satellites and spacecrafts has been addressed to keep the electronic components within the maximum allowable temperature limit. To accommodate high heat flux electronic devices in space equipment, a pulsating heat pipe (PHP) has been gaining much attention as an ideal candidate for space applications due to its simple wickless structure and its long-distance heat transport capability. A thermally-driven two-phase flow inside the PHP is known to transfer heat very efficiently until the heat input is below a certain limit. If the applied heat input exceeds the maximum allowable limit, the thermal performance of the PHP is significantly degraded as a result of the so-called dryout phenomenon. In this study, a systematic investigation is carried out on the heat transport capability of the PHP. As the preliminary study, flow visualization is performed using high-speed photography together with our image-processing algorithm: Based on the flow visualization results, it is postulated that the heat transport capability is significantly affected by the effective void fraction, which is defined as the average length of the vapor region occupied by vapor plugs (enclosed by liquid films) over the effective length of the PHP. This postulate is experimentally confirmed with PHPs with different lengths and various filling ratios. The optimum value for the effective void fraction is shown to decrease with increasing effective length or alternatively the optimum filling ratio increases with increasing effective length. Finally, a correlation of the optimum filling ratio is developed and found to be accurate in predicting the experimental data. This correlation provides a design guideline for choosing the optimum filling ratio for each PHP with a specified length to achieve its highest heat transport capability

Experimental and theoretical studies on oscillation frequencies of liquid slugs in micro pulsating heat pipes

The purpose of this study is to experimentally examine the oscillation frequencies of liquid slugs in multi-turn micro pulsating heat pipes (MPHPs) and to perform a theoretical study for better understanding of the experimental results. A series of experiments on silicon-based MPHPs with different overall lengths of 40, 50, and 60 mm are performed at different heat inputs in a bottom-heating mode. Ethanol is used as a working fluid at a filling ratio of 55%. From a spectral analysis on flow visualization data, dominant frequencies of the MPHPs are identified for each experimental condition. To theoretically estimate the dominant frequencies, a &apos;non-adiabatic&apos; vapor spring liquid mass model is proposed: The spring action of a vapor plug is linked not only to a volume variation but also to a mass variation of a non-adiabatic vapor plug via phase change processes. A distinguishing feature of this model is that it is capable of handling the mass variation of a vapor plug due to phase change processes. Based on the model, a set of parameters related to the oscillation frequency is explicitly determined: the number of turns, channel length, filling ratio, liquid density, vapor pressure and specific heat ratio. A closed-form correlation of the oscillation frequency is proposed and found to be accurate in predicting the experimental data to within 15%. It is also shown that the oscillation frequency is over-predicted by more than 100% of the experimental data when the spring-mass model is used without including the mass variation due to phase change processes

THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON FLOW AND THERMAL CHARACTERISTICS OF MICRO PULSATING HEAT PIPES

In this study, experimental and theoretical investigations are performed to reveal a relationship between heat transfer and flow behavior of plugs/slugs in a micro pulsating heat pipe (MPHP). A silicon-based MPHP with 5 turns and a hydraulic diameter of 667 ??m is fabricated using MEMS techniques. Experiments are performed at various levels of heat input with a bottom-heating mode and ethanol is used as a working fluid at a fixed filing ratio of 55%. Flow visualization is conducted together with a temperature measurement. In the five-turn MPHP, five liquid slugs and five vapor plugs are observed to have harmonic oscillation with two features: First, each liquid slug exhibits a harmonic motion with a phase difference of 2??/5 between adjacent slugs. Secondly, two menisci located at both ends of each vapor plug are observed to be asymmetrically distributed: the time-averaged position of one meniscus (higher-meniscus) is always located higher than that of the other (lower-meniscus). The former and the latter characteristics represent (1) dynamic behavior and (2) static behavior of plugs/slugs in the MPHP, respectively. To find a link between heat transfer and flow behavior, heat transfer to each vapor plug is numerically calculated. From the numerical simulation, the relationship between heat transfer and flow behavior is identified as follows: (1) A nonzero net heat transfer rate to each vapor plug via evaporation/condensation governs dynamic behavior and (2) the heat input determines static behavior. Both dynamic and static behavior may affect the heat transport capability of the MPHP. However, it is static behavior rather than dynamic behavior that determines the maximum heat transport capability of the MPHP. Therefore, a model for the (time-averaged) vapor distribution is suggested and validated with experimental results. The suggested model is shown to be useful for predicting the heat input at which the MPHP attains its maximum thermal performance

Understanding of the thermo-hydrodynamic coupling in a micro pulsating heat pipe

In this study, experimental and theoretical investigations are performed to identify the thermohydrodynamic characteristics of a micro pulsating heat pipe (MPHP). Specifically, the relationship between the heat input and the vapor distribution observed in the MPHP is revealed. A silicon-based MPHP with five turns and a hydraulic diameter of 667 m is fabricated using MEMS techniques. Experiments are performed, using ethanol as a working fluid at a filling ratio of 55%, in a vertical orientation with a bottom-heating mode. Flow visualization is conducted together with a temperature measurement. In the MPHP, two menisci located at both ends of each vapor plug are observed to be asymmetrically distributed: the position of one meniscus (up-header) is always located higher than that of the other (down-corner) and is linearly increased with an increasing heat input. At a critical value of the heat input, the position of the up-header meniscus reaches its upper limit and cannot be increased further. At this upper limit, the thermal performance of the MPHP reaches its maximum and cannot be increased further. This suggests that physically the (asymmetric) vapor distribution is the key factor that determines the heat transport capability of the MPHP. To theoretically explain the relationship between the heat input and the vapor distribution in the MPHP, a model for the asymmetric vapor distribution is developed. Based on the model, a correlation for predicting the positions of vapor menisci is proposed. Finally, the proposed correlation is shown to be useful for predicting the heat input at which the MPHP attains its maximum thermal performance. (C) 2018 Elsevier Ltd. All rights reserved

Characteristics of oscillating flow in a micro pulsating heat pipe: Fundamental-mode oscillation

Theoretical and experimental analyses are performed for oscillating flow in a micro pulsating heat pipe (MPHP). A meandering rectangular micro-channel with a hydraulic diameter of 667 mu m is engraved on a silicon wafer to form a five-turn closed-loop. Flow visualization through a glass top using a high-speed camera is conducted together with temperature measurement for thermal characterization of the MPHP. The MPHP is observed to have a harmonic oscillating motion: each liquid slug in the MPHP is observed to oscillate at frequency ranging from 40 to 50 Hz with a phase difference of 2 pi/5 between adjacent slugs. A closed-form expression for the oscillating motion of the slugs is suggested from a vapor spring-liquid mass model. To quantitatively explain the oscillating mechanism by the vapor spring, a link between the spring motion of the vapor plug and heat transfer to the vapor plug is found: Expansion and contraction of the vapor plug are shown to result from continuous evaporation and condensation at the liquid film enveloping the vapor plug. The evaporation heat transfer and the condensation heat transfer are shown to be out of phase and in turn result in a nonzero net heat transfer rate. To mathematically express the relationship between the net heat transfer rate and the spring motion of the vapor plug, a semi-analytic model is proposed. A semi-analytic expression for the vapor spring constant is developed and validated with experimental results. This study on the fundamental-mode oscillation in MPHP is expected to be used as a building block for investigating more complex oscillating motion in PHPs. (C) 2017 Elsevier Ltd. All rights reserved