An Experimental Investigation on the Influence of Copper Ageing on Flow Boiling in a Copper Microchannel

Modeling of pressure drop and heat transfer for flow boiling in a mini/micro-channel of rectangular cross-section

Abstract: In the present study, a one-dimensional model is proposed to estimate the pressure drop and heat transfer coefficient for flow boiling in a rectangular microchannel. The present work takes into account the pressure fluctuations caused due to the confined bubble growth and the effect of pressure fluctuations on the heat transfer characteristics. The heat transfer model considers five zones, namely, liquid slug, partially confined bubble, fully confined (elongated) bubble, partial dryout and full dryout. The model incorporates the thinning of liquid film due to shear stress at liquid-vapour interface in addition to evaporation. The transient fluctuations in pressure and heat transfer coefficient, along with the time-averaged ones, are verified with the experimental data available in the literature. Heat transfer characteristics with flow reversal caused by inlet compressibility are also presented.

On the prediction of pressure drop in subcooled flow boiling of water

Abstract: Subcooled flow boiling heat transfer is one of the most preferred heat removal methods, especially in areas where high heat fluxes are encountered. The design of heat sinks employing subcooled flow boiling involves prediction of pressure drop, in addition to heat transfer coefficient, as it exerts a huge influence on the local heat transfer, stability of the system and the critical heat flux. This paper provides a systematic review of the performance of the prior correlations and their applicability for different conditions based on a large experimental database compiled from 12 sources containing 1365 data points in total, focussing on water as the working fluid. The results of the assessment are used to identify the right correlation applicable for a specific operating range. New correlations are proposed for the cases and ranges for which the predictability of the prior correlations is unsatisfactory.

On the Prediction of Pressure Fluctuations and Pressure Drop Caused by Confined Bubble Growth During Flow Boiling in a Rectangular Mini/Micro-Channel

Abstract: Heat-sinks based on flow boiling in microchannels have the potential to mitigate temperature rise in high heat flux devices such as electronic equipment. One of the major challenges is the instabil...

Effect of boiling-induced aging on pool boiling heat transfer performance of untreated and laser-textured copper surfaces

Abstract: Despite its importance for practical applications, investigation of aging of enhanced boiling heat transfer surfaces, which occurs during exposure to the boiling process, is often neglected. This study explores the boiling-induced aging behavior of an untreated and a laser-textured copper boiling surface during an eight-day testing period under pool boiling conditions using saturated water at atmospheric pressure. During the test, approx. 40–50 h of intermittent boiling operation were simulated. Boiling curve measurements are used to quantify boiling performance and measurements during steady state operation periods are used to analyze the heat transfer coefficient variations. SEM imaging, contact angle measurements and Raman spectroscopy are used to analyze the surface morphology, chemistry and wettability before and after exposure to hot water and boiling. The results show that no universal prediction of aging effects is possible. A distinct two-stage aging behavior resulting in increased performance was observed on the reference surface, while the boiling performance of the laser-textured surface changed very little during the test. The results also indicate that functionalized surfaces, which might primarily rely on surface microtopography for boiling enhancement, are presumably less prone to boiling behavior changes resulting from oxidation since slight changes in surface wettability will only have a minor effect on the overall boiling performance. Furthermore, evaluation of long-term performance of enhanced surfaces is necessary since their aging behavior differs from untreated surfaces.

Subcooled Flow Boiling Heat Flux Enhancement Using High Porosity Sintered Fiber

Abstract: Passive methods to increase the heat flux on the subcooled flow boiling are extremely needed on modern cooling systems. Many methods, including treated surfaces and extended surfaces, have been investigated. Experimental research to enhance the subcooled flow boiling using high sintered fiber attached to the surface was conducted. One bare surface (0 mm) and four porous thickness (0.2, 0.5, 1.0, 2.0 mm) were compared under three different mass fluxes (200, 400, and 600 kg·m−2·s−1) and three different inlet subcooling temperature (70, 50, 30). Deionized water under atmospheric pressure was used as the working fluid. The results confirmed that the porous body can enhance the heat flux and reduce the wall superheat temperature. However, higher porous thickness presented a reduction in the heat flux in comparison with the bare surface. Bubble formation and pattern flow were recorded using a high-speed camera. The bubble size and formation are generally smaller at higher inlet subcooling temperatures. The enhancement in the heat flux and the reduction on the wall superheat is attributed to the increment on the nucleation sites, the increment on the heating surface area, water supply ability through the porous body, and the vapor trap ability.

Describing the Uncertainties in Experimental Results

Abstract: It is no longer acceptable, in most circles, to present experimental results without describing the uncertainties involved. Besides its obvious role in publishing, uncertainty analysis provides the experimenter a rational way of evaluating the significance of the scatter on repeated trials. This can be a powerful tool in locating the source of trouble in a misbehaving experiment. To the user of the data, a statement (by the experimenter) of the range within which the results of the present experiment might have fallen by chance alone is of great help in deciding whether the present data agree with past results or differ from them. These benefits can be realized only if both the experimenter and the reader understand what an uncertainty analysis is, what it can do (and cannot do), and how to interpret its results. This paper begins with a general description of the sources of errors in engineering measurements and the relationship between error and uncertainty. Then the path of an uncertainty analysis is traced from its first step, identifying the intended true value of a measurement, through the quantitative estimation of the individual errors, to the end objective—the interpretation and reporting of the results. The basic mathematics of both single-sample and multiple-sample analysis are presented, as well as a technique for numerically executing uncertainty analyses when computerized data interpretation is involved. The material presented in this paper covers the method of describing the uncertainties in an engineering experiment and the necessary background material.

CuO Nanowires Can Be Synthesized by Heating Copper Substrates in Air

Abstract: This paper describes a vapor-phase approach to the facial synthesis of cupric oxide (CuO) nanowires supported on the surfaces of various copper substrates that include grids, foils, and wires. A typical procedure simply involved the thermal oxidation of these substrates in air and within the temperature range from 400 to 700 °C. Electron microscopic studies indicated that these nanowires had a controllable diameter in the range of 30−100 nm with lengths of up to 15 μm by varying the temperature and growth time. Electron diffraction and high-resolution TEM studies implied that each CuO nanowire was a bicrystal divided by a (111) twin plane in its middle along the longitudinal axis. A possible mechanism was also proposed to account for the growth of these CuO nanowires.

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

Abstract: This paper is the second of a two-part study concerning two-phase flow and heat transfer characteristics of R134a in a micro-channel heat sink incorporated as an evaporator in a refrigeration cycle. Boiling heat transfer coefficients were measured by controlling heat flux (q″ = 15.9 − 93.8 W/cm2) and vapor quality (xe = 0.26 − 0.87) over a broad range of mass velocity. While prior studies point to either nucleate boiling or annular film evaporation (convective flow boiling) as dominant heat transfer mechanisms in small channels, the present study shows heat transfer is associated with different mechanisms for low, medium and high qualities. Nucleate boiling occurs only at low qualities (xe 0.55) flows dominated by annular film evaporation. Because of the large differences in heat transfer mechanism between the three quality regions, better predictions are possible by dividing the quality range into smaller ranges corresponding to these flow transitions. A new heat transfer coefficient correlation is recommended which shows excellent predictions for both R134a and water.

State of the Art of High Heat Flux Cooling Technologies

Abstract: The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm2) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and ...