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Research Papers: Electronic Cooling

# Single-Phase Microscale Jet Stagnation Point Heat Transfer

[+] Author and Article Information
Gregory J. Michna1

Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180michng@rpi.edu

Eric A. Browne, Yoav Peles, Michael K. Jensen

Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180

1

Corresponding author.

J. Heat Transfer 131(11), 111402 (Aug 26, 2009) (8 pages) doi:10.1115/1.3154750 History: Received January 16, 2009; Revised May 04, 2009; Published August 26, 2009

## Abstract

An investigation of the pressure drop and impingement zone heat transfer coefficient trends of a single-phase microscale impinging jet was undertaken. Microelectromechanical system (MEMS) processes were used to fabricate a device with a $67-μm$ orifice. The water jet impinged on an $80-μm$ square heater on a normal surface $200 μm$ from the orifice. Because of the extremely small heater area, the conjugate convection-conduction heat transfer process provided an unexpected path for heat losses. A numerical simulation was used to estimate the heat losses, which were quite large. Pressure loss coefficients were much higher in the range $Red,o<500$ than those predicted by available models for short orifice tubes; this behavior was likely due to the presence of the wall onto which the jet impinged. At higher Reynolds numbers, much better agreement was observed. Area-averaged heat transfer coefficients up to $80,000 W/m2 K$ were attained in the range $70. This corresponds to a $400 W/cm2$ heat flux at a $50°C$ temperature difference. However, this impingement zone heat transfer coefficient is nearly an order-of-magnitude less than that predicted by correlations developed from macroscale jet data, and the dependence on the Reynolds number is much weaker than expected. Further investigation of microjet heat transfer is needed to explain the deviation from expected behavior.

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## Figures

Figure 1

Schematic of the flow loop used in the experiments

Figure 2

Schematic of the assembly of the fixture, the microdevice and the cover plate

Figure 3

(a) Schematic of the microdevice and (b) a close up view of the orifice and the heater. The jet issues from the orifice in the center of the bottom surface of the channel. It impinges upon the heater 200 μm above (on the bottom surface of the Pyrex wafer), and the fluid exits down from either end of the channel.

Figure 4

Schematic showing the paths from the heater through which heat is lost. Most of the heat is lost through the path labeled Qloss,2, which cannot be measured independently or calculated without knowledge of local heat transfer coefficients.

Figure 5

Relationship between the resistance of the heater and average heater surface temperature. The error bars are smaller than symbol size.

Figure 6

Plot of the measured vacuum heat losses and the heat losses calculated using finite element analysis for several values of heat transfer coefficient. The error bars for the measured values are smaller than the symbol size.

Figure 7

The effect of Reynolds number on the pressure loss coefficient

Figure 8

The heat transfer performance of the microjet

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