Cavitation in MicroElectroMechanical Systems (MEMS): Importance, Deviations from Conventional Scale, and Preliminary Results

ABSTRACT Hydrodynamic cavitation in micro systems is a fundamental engineering problem that is poorly understood. The lack of knowledge on cavitation relevant to MEMS devices is impeding the practical realization of novel high-velocity microfluidic machines. This paper divulges differences between cavitation occurring inside micro and conventional systems, and also indicates the limited applicability of conventional knowledge to predict and understand cavitating flows in microdomains. A detailed discussion delineating the possible reasons of such a divergence is presented in this article. Additionally, selected results obtained from preliminary experiments on cavitation in micro-domains are presented

CROSS FLOW BOILING IN MICRO-PIN FIN HEAT SINKS

ABSTRACT Flow boiling of water across a bank of circular staggered micro pin fins, 250 µm long and 100 µm diameter with pitch-todiameter ratio of 1.5, was experimentally studied for mass fluxes ranging from 346 kg/m 2 s to 794 kg/m 2 s and surface heat fluxes ranging from 20 W/cm 2 to 350 W/cm 2 . The local twophase heat transfer coefficients were measured using thermistors located along the flow path of the channel. The flow was visualized and classified as vapor slug and annular flow patterns. Based on the observed flow patterns, the dominant heat transfer mechanism during boiling process was assumed to be convective boiling. INTRODUCTION Single-phase [1-5] and two-phase [6-9] flows in microchannels have been a topic of extensive studies over the last several years. Recently, pin fin microchannel heat sinks have also been studied, but primarily in the context of singlephase flo

The effect of area ratio on microjet array heat transfer

a b s t r a c t The heat transfer performance of five submerged and confined microjet arrays using air and deionized water as the working fluids was investigated. Both inline and staggered array arrangements of jet with diameters of 54 and 112 lm were investigated, and the area ratio (total area of the jets divided by the surface area) was varied between 0.036 and 0.35. Reynolds numbers defined by the jet diameter were in the range of 180-5100 for air and 50-3500 for water. A heat flux of 1100 W/cm 2 was obtained at a fluid inlet-to-surface temperature difference of less than 30°C. The results were compared with established correlations, and no evidence was found to suggest that the behavior of submerged and confined jets at the microscale is fundamentally different than at the macroscale. Reynolds number, Prandtl number, and area ratio were found to significantly affect the heat transfer performance, and a curve fit was developed, which correlated 290 of the 295 data points within ±25% with an MAE of 11%

Subcooled Flow Boiling In A Microchannel With A Pin Fin And A Liquid Jet In Crossflow

Abstract This experimental study presents subcooled flow boiling of an engineering fluid - HFE-7000 - downstream a single pin fin in a microchannel. A liquid secondary jet was introduced into the flow to examine its merits pertinent to heat transfer enhancement. It was found that for HFE-7000 high wall superheats (ΔT(sat,ONB)∼40°C) were required for the onset of nucleate boiling (ONB). Once boiling started, nucleate boiling dominated. Heat transfer coefficient increased monotonically with heat flux, independent of mass flux and jet injections. Secondary flow injection, which was previously found to be an affective single phase heat transfer enhancement technique, showed limited potential for fully developed nucleate boiling

Spatiotemporally Resolved Heat Transfer Measurements For Flow Boiling In Microchannels

Spatiotemporally resolved wall heat transfer measurements can provide valuable insight into the fundamental mechanisms affecting flow boiling in microchannels. Operating at the microscale, necessitates resolving changes in local and instantaneous heat transfer characteristics on the order of 100 μm and 1 kHz, respectively. Straightforward interpretation of transient temperature measurements is often challenging due to the conjugate conduction effects in the substrate, which can dampen the measured and inferred heat transfer quantities. These damping effects are described using a slip coefficient (S), which represents the fraction of the change in the local heat transfer that is registered by a sensor (negligible thermal mass) located on a given substrate. Using S, arguments are presented that the conduction patterns in the substrate are predominantly 1-D (i.e. into the substrate) at suitable spatiotemporal-scales. Building on these fundamental considerations, a numerical procedure is adopted to allow a time varying estimate of the local convective heat flux and heat transfer coefficient from transient temperature measurements. Examples of this framework are showcased with experimental results and discussions for interactions observed during flow boiling of HFE 7000 in a single microchannel (hydraulic diameter = 370 μm). At the relatively low mass flux of 200 kg/m2 s reported in this work, liquid evaporation was found to dictate the local heat transfer trends. High rates of heat transfer were observed to accompany the growth of bubbles and evaporation of the liquid film under vapor slugs. Local dryout was routinely observed in the bubbly and slug flow regime and found to initially enhance heat transfer (i.e. at the creation and subsequent propagation of the three-phase contact line) and present near-zero heat transfer rates in the dried-out domain

Challenges In Micro-Channel Heat Transfer Experiments: Insight On Conjugate Heat Transfer Effects

A numerical study was performed to obtain insight into conduction/convection conjugate heat transfer processes during a typical experiments performed at the micro scale. The experimental work of Wang et al. [1] was used as a baseline to quantify such effects. It consisted of a 225 μm × 18.5 mm × 1.5 mm microchannel with a pillar, which had jet slits. Commercially available software package, Star CCM+, was used for the simulations. Fluid, solid and heater regions were modeled together with heat transfer interfaces at the contact surfaces. Reynolds number of 123, 200, and 280 were simulated. Each case includes jet introduction to the flow quantified with momentum coefficient of 3%, 5%, and 10%. The numerical model was verified by the Grid Convergence Index methods using three different grids and agreed with experimental results within a 10% discrepancy. The discrepancies between numerical and experimental results were found in terms of heat transfer distribution and heat transfer coefficients, which were mainly due to inevitable assumptions made while post processing the experimental data. Therefore, the main aim of this study is to decouple the heat transfer processes inside the three different regions and to provide guidance for future micro scale experimental studies in terms of experimentally non-measurable parameters, such as heat flux paths, local heat transfer coefficients, local boundary heat fluxes, and local temperature values on the heater surfaces