R. A. Wirtz

In-plane effective thermal conductivity of plain-weave screen laminates

A simple-to-fabricate woven mesh, consisting of bonded laminates of two-dimensional plain-weave conductive screens is described. Geometric equations show that these porous matrices can be fabricated to have a wide range of porosity and specific surface area, /spl beta/. A heat transfer model is developed. It shows that the laminates can have a highly anisotropic thermal conductivity vector, with in-plane effective thermal conductivities ranging up to 78.5% of base material values. A technique to measure the laminate in-plane effective thermal diffusivity is described. Measurements of the in-plane effective thermal diffusivity of copper plain-weave laminates are used to benchmark the model.

Enhanced Heat Transfer/Pressure Drop Measured From a Flat Surface in a Grooved Channel

In the present work, the effect of passive flow destabilization on augmenting the heat transfer/pumping power performance of a rectangular channel is studied experimentally. The goal is to determine how shear layers that span transverse grooves on one wall trigger flow instabilities and effect heat transfer from the opposite surface. This technical note addresses the potential effectiveness of destabilization in gas flows. Pressure drop measurements are made to allow comparisons of different passage configurations on an equal pumping power basis, and measurements are made in the fully developed as well as developing thermal regimes.

A Hybrid Thermal Energy Storage Device, Part 1: Design Methodology

is developed. The stabilization time and maximum operating (hot side) temperature-totransition temperature difference are used to characterize the performance of the heat sink. The thermal properties of the PCM employed in the design are investigated. Integration of a design optimization algorithm into a thermal performance model of the TES-hybrid heat sink results in determination of a best design subject to geometric and heat loading constraints. A prototype based on this best design is build and used to benchmark the performance model. The performance measured is consistent with the simulation model predictions of performance.@DOI: 10.1115/1.1646419#

Cylindrical pin-fin fan-sink heat transfer and pressure drop correlations

New pressure drop/flow rate measurements coupled with a previously reported heat transfer database have resulted in new friction factor and heat transfer correlation equations for fan-driven impinging flow through square arrays of cylindrical cross section pin-fins. The correlation equations include the effect of coolant flow rate, pin-fin density and pin-fin height. A comparison with a similar jet-driven impingement flow configuration shows that the heat transfer/pressure drop characteristics. of the two flow configurations are fundamentally different. When the same arrays are compared at the same coolant flow rate, the fan-driven configuration will result in a lower pressure drop and heat transfer rate. An analysis shows that different optimal pin-fin configurations (fin height and fin density) result, depending on the design criteria imposed on the flow.

A Hybrid Thermal Energy Storage Device, Part 2: Thermal Performance Figures of Merit

Two figures of merit for hybrid Thermal Energy Storage (TES) units are developed volumetric figure of merit, V ̃ , and the temperature control figure of merit, DT̃. A dimensional analysis shows that these quantities are related to the performance specificat the storage unit and its physical design. A previously benchmarked semi-empirical volume model is used to study the characteristics of various plate-type TES-unit de A parametric study is used to create a database of optimal designs, which is then u form simple correlations of V ̃ and DT̃ in terms of design requirements and attributes. preliminary design procedure utilizing these figures of merit is suggested. Sample c lations show that these correlations can be used to quickly determine the design attr of a plate-type TES-unit, given design requirements. @DOI: 10.1115/1.1646420 #

Augmented Heat Transfer in a Recovery Passage Downstream From a Grooved Section: An Example of Uncoupled Heat/Momentum Transport

Earlier experiments have shown that cutting transverse grooves into one surface of a rectangular cross-sectional passage stimulates flow instabilities that greatly enhance heat transfer/pumping power performance of air flows in the Reynolds number range 1000 < Re < 5000. In the current work, heat transfer, pressure, and velocity measurements in a flat passage downstream from a grooved region are used to study how the flow recovers once it is disturbed. The time-averaged and unsteady velocity profiles, as well as the heat transfer coefficient, are dramatically affected for up to 20 hydraulic diameters past the end of the grooved section. The recovery lengths for shear stress and pressure gradient are significantly shorter and decrease rapidly for Reynolds numbers greater than Re = 3000. As a result, a 5.4-hydraulic-diameter-long recovery region requires 44 percent less pumping power for a given heat transfer level than if grooving continued.

Correlation of Subatmospheric Pressure, Saturated, Pool Boiling of Water on a Structured-Porous Surface

Saturated pool-boiling experiments at 1 atm and subatmospheric pressure assess the utility of fine-filament screen-laminate enhanced surfaces as effective bubble nucleation sites. Experiments were conducted on vertically oriented, multilayer laminates in saturated distilled water at pressures of 0.2―1.0 atm. The performance of 12 different copper-filament surfaces, having pore hydraulic diameters ranging from 14 μm to 172 μm, is documented. Experimental results show that boiling performance is a strong function of screen-laminate geometry. In the present work, enhancement of up to 27 times that of an unenhanced surface was obtained at a superheat of 8 K and a pressure of 0.2 atm. Dimensional analysis and multiparameter regression are used to develop a heat transfer correlation that relates the boiling heat transfer coefficient to the lamination geometry.

Three-Dimensional Simulations of Enhanced Heat Transfer in a Flat Passage Downstream From a Grooved Channel

Navier-Stokes simulations of three-dimensional flow and augmented convection in a flat passage downstream from a fully developed channel with symmetric, transverse grooves on two opposite walls were performed for 405 < Re < 764 using the spectral element technique. Unsteady flow that develops in the grooved region persists several groove-lengths into the flat passage, increasing both local heat transfer and pressure gradient relative to that in a steady flat passage. Moreover, the heat transfer for a given pumping power in the first three groove-lengths of the flat passage was even greater than the levels observed in a fully developed grooved passage.

Thermal and Mechanical Characteristics of a Multifunctional Thermal Energy Storage Structure

Thermal energy storage (TES) sandwich-structures that combine the heat storage function with structural functionality are described. The structure consists of a thermal interface (TI) connected to a hollow plate lamination. Each laminate is a hollow aluminum plate having a series of mm-scale channels or compartments that are filled with phase change material (PCM). Heat storage is via the latent heat of the PCM. A generalized thermal response model that is applicable to a wide range of channel geometrical configurations is described. The model couples the thermal response of the TI to the hollow aluminum plate/PCM-volume. The temporal response of the system is easily obtained via numerical solution of two ordinary differential equations, which can be solved to give closed-form solutions subject to a simple assumption. Thermal analysis delineates geometrical configurations that have good thermal response characteristics. The mechanical properties of the laminated structure are determined experimentally. Four-point bending experiments are conducted using specimens made with three layers of hollow plates laminated using a structural adhesive. An energy method is developed to model both the deformation and strength of the laminated structure. The energy method is developed based on the assumption that plane cross sections of the structure remain plane under bending, a condition that is valid for both linear and nonlinear materials. The energy method can provide deformation of the aluminum laminates comparable with the experiments. Experiments and modeling indicate that these laminated structures have an excellent performance-to-weight ratio.

Investigation of plasma evolution in a coaxial small-gap magnetically insulated transmission line

Interferometry and two-frame schlieren imaging were used to study arc discharge evolution in a small-gap, coaxial, magnetically insulated transmission line driven by a 2-TW generator with a current pulse rise time of 70 ns. Two kinds of plasma objects were observed in experiments: plasma of arc discharges and low-density peripheral plasma. Plasma fills most of the magnetically insulated transmission line (MITL) gap in the area of the arc and produces a stripe trace of evaporated metal on the surface of electrodes. Arc discharge typically arises near the cathode. Anode plasma arises in the later stage, after which, the plasma fills the gap. A scenario of plasma evolution of the arc discharge is discussed. Low-density plasma is located in thin layers near the cathode or the anode. It plays a role in the seeding of arc discharges that grow before the closure of the gap and dissipates after the closure.