IMECE2008-69051 EXPERIMENTAL STUDY OF ADIABATIC WATER LIQUID-VAPOR TWO-PHASE PRESSURE DROP ACROSS AN ARRAY OF STAGGERED MICRO-PIN-FINS

ABSTRACT This study concerns pressure drop of adiabatic water liquid-vapor two-phase flow across an array of 1950 staggered square micro-pin-fins having a 200×200 micron cross-section by a 670 micron height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. An inline immersion heater upstream of the micro-pin-fin test module was employed to produce liquid-vapor two-phase mixture, which flowed across the micro-pin-fin array. The test module was well insulated to maintain an adiabatic condition. Four maximum mass velocities of 184, 235, 337, and 391 kg/m²s, and a range of vapor qualities for each maximum mass velocity were tested. Measured pressure drop increases drastically with increasing vapor quality. Nine existing twophase pressure drop models and correlations were assessed. The Lockhart-Martinelli correlation for laminar liquid-laminar vapor combination in conjunction with a single-phase friction factor correlation proposed for the present micro-pin-fin array provided the best agreement with the data

A Thin-Film Heat Flux Sensor Fabricated on Copper for Heat Transfer Measurements in Parallel Channel Heat Sinks

A combination of lithography-based microfabrication and micro end milling is used to manufacture thin-film resistance temperature detector (RTD) heat flux sensors on bulk copper substrates. The fabrication process uses photoresist patterning, metal deposition, and lift-off to build the sensor and micro-end milling to segment the sensors. Micro end milling tests were performed to establish determine the optimum conditions for sensor removal which minimized delamination and burr formation. It was determined that starting on the backside (opposite the sensor) of the copper wafer and machining through to the thin film layers resulted in the least amount of burr formation

Transport phenomena in single-phase and two-phase micro-channel heat sinks

Various transport phenomena in micro-channel heat sinks were investigated in this thesis work. Included are single-phase pressure drop and heat transfer, incipient boiling heat flux, two-phase flow instability, flow patterns, flow boiling heat transfer, flow boiling pressure drop, and critical heat flux (CHF). A micro-channel heat sink experimental facility was designed and constructed. The micro-channel heat sink was fabricated from oxygen-free copper and contained twenty-one parallel rectangular 231 x 713 μm (215 x 821 μ m for critical heat flux experiments) micro-channels. Deionized water was the working liquid. Single-phase pressure drop and heat transfer were studied experimentally and numerically, with excellent agreement. Experiments were performed to measure incipient boiling heat flux. A mechanistic model based on bubble departure criterion was developed, whose predictions showed good agreement with the experimental data. Two types of two-phase hydrodynamic instability were identified: severe pressure drop oscillation and mild parallel channel instability. The former was eliminated by throttling a control valve upstream of the heat sink. Moderate to high heat fluxes produced mostly annular flow. Experiments were also conducted on flow boiling heat transfer and pressure drop. Previous heat transfer correlations were assessed and deemed inaccurate. Previous two-phase pressure drop correlations were also examined, and a new correlation was proposed which shows better accuracy than prior correlations. An annular flow model was developed, incorporating unique features of two-phase micro-channel flow. Good agreement was achieved between the model predictions and experimental data for both flow boiling heat transfer and pressure drop. Experimental critical heat flux (CHF) was found to be independent of inlet temperature, but CHF increased with increasing mass velocity. A new CHF correlation was proposed for two-phase micro-channel heat sink that shows excellent accuracy in predicting existing heat sink data. Based on the fundamental understanding of the various transport phenomena associated with two-phase micro-channel flow, a comprehensive methodology was developed for optimizing the design of a two-phase micro-channel heat sink. The proposed optimization methodology yields an acceptable design region encompassing all possible micro-channel dimensions corresponding to a prescribed coolant flow rate or pressure drop

Collapsing behavior of spark-induced cavitation bubble in rigid tube

The phenomenon of cavitation within tubes is a common scenario in the fields of medicine and industry. This paper focuses on the effects of rigid circular tube length, diameter and the distance of bubble - tube port on the behavior of bubble in tube. The low-voltage discharge technique was utilized to induce a cavitation bubble in deionized water. The effects of rigid tube lengths, diameters, and bubble-tube port distances on the morphology of bubbles are observed using high-speed camera. It has been found that as the length of the rigid tube increases, so does the period, and this effect is more pronounced in tubes with smaller diameters. Conversely, the cavitation bubble period decreased and then stabilized as the tube diameter increased, the ratio of tube radius and the bubble radius exceeds 4.8, the period of bubble in tube is similar to that of bubble in free field. Further analysis of the influence of tube characteristics on microjets reveals that a pair of oppositely microjets were formed along the tube axis by the bubble near the midpoint of the tube axis. Moreover, when the non-dimensional tube length η < 3.5, the increase tube diameter results in a decrease microjet velocity. It has also been observed that as the bubble gradually approaches the interior of the tube, the velocity of microjets directed inward decreases. Additionally, the smaller the diameter of the tube, the greater the bubble-tube port distance required for the microjets to reach the same level of velocity as bubble near the center of the tube axis. These findings hold theoretical implications for improvement of targeted drug delivery efficiency in medicine and enhance the operational efficiency of inertial micropumps in industries