Search

This paper presents an investigation of forced convection effects on transient boiling heat transfer of water on plate-type heaters, at atmospheric pressure, under exponentially escalating heat fluxes. It complements the work performed under pool boiling conditions presented in the companion paper (Part I). Infrared (IR) thermometry and high-speed video (HSV) were used to gain insight into the physical phenomena and generate data that can be used for development and validation of accurate models of transient flow boiling heat transfer. Exponential power escalations with periods in the range from 5 to 500 ms, and subcooling of 10, 25 and 75 K were explored. The Reynolds number was varied from 25,000 to 60,000, depending on the subcooling. Single-phase heat transfer, onset of the boiling driven (OBD) heat transfer regime and overshoot (OV) conditions were identified. The experimental data suggest that during the single-phase heat transfer regime, forced convection is the dominant heat transfer mechanism for long periods, whereas transient conduction is more important for short periods. A criterion based on the normalized time scale for turbulent heat transfer is shown to capture all single-phase heat transfer data on a single curve. At a given period, OBD heat flux and wall superheat, as well as OV wall superheat increase with increasing subcooling and increasing Reynolds number. For a given Reynolds number and a given subcooling, they decrease with increasing periods at short periods, when the dominant heat transfer mechanism is transient conduction, whereas they barely change for long periods, as the dominant single-phase heat transfer mechanism is forced convection. Once boiling is fully developed, the heat transfer coefficient is proportional to the wall superheat to the fourth power and increases with increasing subcooling. [ABSTRACT FROM AUTHOR]

Lack of understanding of flow boiling behavior in reduced gravity poses a major challenge to the development of future space vehicles utilizing two-phase thermal control systems (TCSs). A cost effective method to investigating the influence of reduced gravity on flow boiling is to perform ground experiments at different orientations relative to Earth gravity. This paper is the first part of a two-part study aimed at exploring flow boiling mechanisms of FC-72 in a rectangular channel heated along one wall or two opposite walls. Experiments are performed in vertical upflow, vertical downflow and horizontal flow, subject to large variations in mass velocity, inlet quality and wall heat flux. Detailed measurements are used to investigate the influences of orientation, and therefore gravity, on boiling curve, local and average heat transfer coefficients, and pressure drop, and their relationship with interfacial behavior is captured with high-speed video. For horizontal flow, the effects of gravity are reflected in appreciable stratification across the channel at low mass velocities, with gravity aiding vapor removal from, and liquid return to the bottom heated wall, while accumulating vapor along the top heated wall. For vertical upflow and vertical downflow, with both single-sided and double-sided heating, there is far better symmetry in vapor formation along the channel. The heat transfer coefficient shows significant variations among the different orientations and heating configurations at low mass velocities, but becomes insensitive to orientation above 800 kg/m 2 s, proving inertia around this mass velocity is effective at negating any gravity effects. For low mass velocities, pressure drops are fairly equal for vertical upflow and vertical downflow, but greater than for horizontal flows. However, fairly equal pressure drops are achieved at high mass velocities for all orientations. Overall, this study proves that gravity effects on two-phase pressure drop and two-phase heat transfer are dictated mostly by mass velocity and, to a lesser extent, by inlet quality. [ABSTRACT FROM AUTHOR]

• Raj et al.'s gravity scaling parameters can't be applicable to the controlled wall heat flux condition. • Microgravity boiling mechanisms on chip S 2 × 2 transform from the SDB to BDB regime. • m BDB for controlled wall temperature condition is larger than that for controlled wall heat flux condition. • m BDB for controlled wall heat flux boiling is affected by both T * and L h / L c. • Gravity scaling parameters for controlled wall heat flux boiling were developed. With the ability of dissipating large amount of latent heat in nearly isothermal process, pool boiling heat transfer is an efficient solution for cooling problem of different sized electronic devices in different gravities. For the practical heat dissipation of electronic device usually occurs at controlled wall heat flux condition, it is essential to reveal pool boiling heat transfer characteristics at different sizes and gravities under controlled wall heat flux condition. In this paper, boiling heat transfer mechanisms on two sized smooth silicon chips (chip S 1 × 1 and chip S 2 × 2) in ground gravity and microgravity were studied. It was found that the gravity scaling parameters developed under controlled wall temperature condition cannot be applicable to the controlled wall heat flux condition. Under controlled wall heat flux condition, the microgravity boiling heat transfer performance on chip S 1 × 1 is in surface tension dominated boiling (SDB) regime in the whole heat flux range. While for chip S 2 × 2, it is in SDB regime when heat flux is smaller than 6.44 W·cm−2, but enters into buoyancy dominated boiling (BDB) regime when heat flux is larger than 6.44 W·cm−2. The critical point of the two boiling mechanisms occurs when the threshold gravitational acceleration equals to the experimental microgravity. In BDB regime, the power law coefficient reflecting the gravity effect on nucleate boiling heat transfer performance (m BDB) for controlled wall temperature condition is larger than that for controlled wall heat flux condition, indicating a greater impact of gravity level under controlled wall temperature condition. Moreover, m BDB for controlled wall temperature condition is affected only by non-dimensional wall temperature T *. While for controlled wall heat flux condition, m BDB increases with increasing T * but decreases with increase in heater size, and heater size has a greater influence on m BDB. Based on the available data, the gravity scaling parameters for controlled wall heat flux boiling were developed, which can accurately predict the microgravity boiling heat transfer performance on two sized chips. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the experimental study on transient heat transfer at boiling and development of crisis phenomena in the falling film of subcooled liquid at stepwise heat generation. Experimental data with a high temporal and spatial resolution were obtained with the use of high-speed infrared thermometry and high-speed video recording synchronized simultaneously. It is shown that at vapor bubble appearance and condensation the convective streams are formed there, which lead to a decrease in temperature in the area behind the active nucleation site and local increase in heat transfer intensity. At the same time on the free surface of liquid film the so-called “temperature waves” propagating along the liquid flow are formed. The boiling curves were obtained on the basis of experimental data, and comparison with the known models for description of heat transfer at pool boiling of saturated liquid was performed. It is shown that mechanism of appearance and development of crisis phenomena depended significantly on the Reynolds numbers. Crisis development at high Reynolds numbers relates to dry spot formation at quasi steady-state boiling and their following expansion by the mechanism of longitudinal heat conductivity in the heating wall. The wave development on the free surface at low Reynolds number (Re =601), studied in current research, leads to the breakdown of the film without nucleate boiling due to the action of thermocapillary forces. The critical heat fluxes under these conditions are significantly lower than the critical values obtained for high Reynolds numbers at boiling in the falling films of subcooled water. [Copyright &y& Elsevier]

Abstract: EHD effects on bubble nucleation and its subsequent growth dynamic on a small heater in pool boiling of R113 are investigated experimentally in this paper. The small heater, 5mm×5mm in size, is made of an ITO film consisting of Pyrex glass and ITO-coatings. A non-uniform electric field is generated by a vertical needle at high potential and the small heater at low potential as electrodes. A high-speed camera is used to record the changing shape of a single R113 vapor bubble during its nucleation and its subsequent rise from the small heater at different heat fluxes with and without an externally imposed electric field. Effects of electric field strength and imposed heat flux on the bubble growth time, waiting time, departure frequency, dynamic contact angle and movement of triple contact line are presented. It is found that the electric field enhances nucleate boiling heat transfer of a single R113 bubble in the present experiment. [Copyright &y& Elsevier]