Matthew J. Rau

Boiling Heat Transfer From an Array of Round Jets With Hybrid Surface Enhancements

The effect of a variety of surface enhancements on the heat transfer achieved with an array of impinging jets is experimentally investigated using the dielectric fluid HFE-7100 at different volumetric flow rates. The performance of a 5 × 5 array of jets, each 0.75 mm in diameter, is compared to that of a single 3.75 mm diameter jet with the same total open orifice area, in single-and two-phase operation. Four different target copper surfaces are evaluated: a baseline smooth flat surface, a flat surface coated with a microporous layer, a surface with macroscale area enhancement (extended square pin–fins), and a hybrid surface on which the pin–fins are coated with the microporous layer; area-averaged heat transfer and pressure drop measurements are reported. The array of jets enhances the single-phase heat transfer coefficients by 1.13–1.29 times and extends the critical heat flux (CHF) on all surfaces compared to the single jet at the same volumetric flow rates. Additionally, the array greatly enhances the heat flux dissipation capability of the hybrid coated pin–fin surface, extending CHF by 1.89–2.33 times compared to the single jet on this surface, with a minimal increase in pressure drop. The jet array coupled with the hybrid enhancement dissipates a maximum heat flux of 205.8 W/cm2 (heat input of 1.33 kW) at a flow rate of 1800 ml/min (corresponding to a jet diameter-based Reynolds number of 7800) with a pressure drop incurred of only 10.9 kPa. Compared to the single jet impinging on the smooth flat surface, the array of jets on the coated pin–fin enhanced surface increased CHF by a factor of over four at all flow rates.

Axisymmetric wall jet development in confined jet impingement

The flow field surrounding an axisymmetric, confined, impinging jet was investigated with a focus on the early development of the triple-layered wall jet structure. Experiments were conducted using stereo particle image velocimetry at three different confinement gap heights (2, 4, and 8 jet diameters) across Reynolds numbers ranging from 1000 to 9000. The rotating flow structures within the confinement region and their interaction with the surrounding flow were dependent on the confinement gap height and Reynolds number. The recirculation core shifted downstream as the Reynolds number increased. For the smallest confinement gap height investigated, the strong recirculation caused a disruption of the wall jet development. The radial position of the recirculation core observed at this small gap height was found to coincide with the location where the maximum wall jet velocity had decayed to 15% of the impinging jet exit velocity. After this point, the self-similarity hypothesis failed to predict the evoluti...

An Experimental Study of a Multi-Device Jet Impingement Cooler With Phase Change Using HFE-7100

Jet impingement cooling with phase change has shown the potential to meet the increased cooling capacity demands of high-power-density (of order 100 W/cm2) automotive electronics components. In addition to improved heat transfer, phase change cooling has the potential benefit of providing a relatively isothermal cooling surface. In the present study, two-phase jet impingement cooling of multiple electronic devices is investigated, where the fluorinated dielectric fluid HFE-7100 is used as the working fluid. Four different types of jet arrays, namely, a single round jet with orifice diameter of 3.75 mm, and three different 5 × 5 arrays of round jets with orifice diameters of 0.5 mm, 0.6 mm and 0.75 mm, were tested and compared for both heat transfer and pressure drop. The experimental Reynolds number at the orifice ranged from 1860 to 9300. The results show that for the same orifice pressure drop, the single jet reached CHF at approximately 60 W/cm2, while the 5 × 5 array (d = 0.75 mm) safely reached heat fluxes exceeding 65 W/cm2 without reaching CHF. Additionally, the experimental results show that the multi-device cooler design causes an unintended rise in pressure inside the test section and a subsequent increase in sub-cooling from 10 K to 23.3 K.Copyright

An Experimental Study of a Single-Device Jet Impingement Cooler With Phase Change Using HFE-7100 and a Vapor Extraction Manifold

There is substantial ongoing research into jet impingement cooling with phase change for high heat flux electronics applications. Higher heat transfer coefficients can be achieved through coolant phase change, although the proper evacuation of the resulting two-phase flow is important as it can affect the overall heat transfer performance of the cooler. In prior work, the accumulation of vapor in a multi-device cooler during the two-phase heat transfer process was shown to cause a build-up of pressure inside the cooler. This increase in pressure is logically related to the position of the cooler inlet and outlet ports with respect to the internal cooling geometry. Such pressure increases lead to an increase in the saturation temperature of the coolant and additional concerns regarding fluid containment. The present study describes a novel two-phase single-device cooler with HFE-7100 as the coolant, where the design allows for efficient removal of vapor from the test-section via a sloped outlet manifold. The performance of the cooler was evaluated using smooth and finned copper heat spreaders. To assess the effectiveness of the vapor extraction manifold, a comparison is made with the performance of a related multi-device cooler. Experimental results show that the single-device design reduces pressure build-up inside the cooler by an order of magnitude from 59 kPa to 7 kPa. A 36% increase in the effective heat transfer coefficient (∼19,000 W/m2K) at 50 W/cm2) was also achieved using the new single-device design with the smooth heat spreader when compared to the multi-device cooler. Additionally, by enhancing the heat spreader surface area with fins, the effective heat transfer coefficient was further boosted to 23,000 W/m2K.Copyright

Visualization of Confined Jet Impingement With Boiling Using Time-Resolved Stereo-PIV

Two-phase liquid-vapor flow field measurements of confined jet impingement with boiling are performed using time-resolved stereo particle image velocimetry (stereo-PIV). A single circular jet of water, impinges normally from a 3.75 mm-diameter orifice onto a submerged circular heat source at an orifice-to-target spacing of 4 jet diameters. The impinging jet outflow including the vapor generated at the heat source are confined between the jet orifice plate and the bottom test section wall. Fluorescent seeding particles (10 μm in diameter) and time-resolved PIV measurements (taken at a sampling rate of 750 Hz) allow for imaging of the instantaneous interactions between the liquid and vapor structures. Liquid-phase velocity vectors within the two-phase flow field (with high vapor fractions) are presented as a function of heat flux at jet Reynolds numbers of 5,000 and 15,000 and contrasted with single-phase flow. The time-resolved measurements are used to highlight the influence of the vapor phase on the liquid flow field. It is found that bubble formation effectively blocks the developing wall-jet flow on the heated surface. The resulting liquid flow field in the confinement gap is dominated by vapor motion rather than by the entrainment from the developing wall jet.Copyright