Frank M. Gerner

GENERAL FILM CONDENSATION CORRELATIONS

A comprehensive film condensation heat transfer correlation, established on the basis of analytical and empirical results from the literature, is in excellent agreement with all existing data. It incorporates the effects of interfacial shear stress, interfacial waviness, and turbulent transport in the condensate film. The usefulness of this correlation is demonstrated for annular-film condensation inside tubes. This correlation can easily be incorporated into condensing models once the interfacial shear stress is known. Two cases, cocurrent annular-film condensation inside vertical and horizontal tubes and countercurrent annular-film reflux condensation such as occurs inside the two-phase closed thermosyphon, are used to demonstrate this procedure.

Natural convection in partitioned air-filled rectangular enclosures

Abstract This work is a study on the effect a vertical partition has on steady-state natural convection in air-filled rectangular enclosures. A finite-difference scheme was used to solve the governing equations. Computed Nusselt numbers are presented as a function of the governing parameters. It was found that placing a partition midway between the vertical walls of an enclosure produces the greatest reduction in heat transfer and often compares favorably with fully insulating the enclosure with a porous material.

Effect of Thermal Deformation on Part Errors in Metal Powder Based Additive Manufacturing Processes

In metal additive manufacturing (AM) processes, parts are manufactured in layers by sintering or melting metal or metal alloy powder under the effect of a powerful laser or an electron beam. As the laser/electron beam scans the powder bed, it melts the powder in successive tracks which overlap each other. This overlap, called the hatch overlap, results in a continuous cycle of rapid melting and resolidification of the metal. The melting of the metal from powder to liquid and subsequent solidification causes anisotropic shrinkage in the layers. The thermal strains caused by the thermal gradients existing between the different layers and between the layers and the substrate leads to considerable thermal stresses in the part. As a result, stress gradients develop in the different directions of the part which lead to distortion and warpage in AM parts. The deformations due to shrinkage and thermal stresses have a significant effect on the dimensional inaccuracies of the final part. A three-dimensional thermomechanical finite element (FE) model has been developed in this paper which calculates the thermal deformation in AM parts based on slice thickness, part orientation, scanning speed, and material properties. The FE model has been validated and benchmarked with results already available in literature. The thermal deformation model is then superimposed with a geometric virtual manufacturing model of the AM process to calculate the form and runout errors in AM parts. Finally, the errors in the critical features of the AM parts calculated using the combined thermal deformation and geometric model are correlated with part orientation and slice thickness.

EXPERIMENTAL RESULTS FOR LOW-TEMPERATURE SILICON MICROMACHINED MICRO HEAT PIPE ARRAYS USING WATER AND METHANOL AS WORKING FLUIDS

An experimental test facility was constructed to test and verify the operation of two parallel arrays of anisotropicalfy micromachined (etched) micro heal pipes (MHPs) on a single crystalline (100) semiconductor silicon wafer, A micro heat pipe is a small-scale device used to transport energy from a heat source to a heat sink in nearly isothermal operation. The individual MHP was an isosceles triangle with a length of 25.4 mm, and a width of 100 /xm and 260 fim for the arrays. A 7740 Pyrtx glass wafer was used to seal the pipe array hermetically. The transparent glass allowed visual inspection of the level of the working fluid filling the pipe array. Two working fluids were tested, pure water and methanol. A Kapton heater was used to supply the heat input, and cooling water flowing through the condenser (to remove the applied heat) was used as a heat sink. Filling charges of 5%, 10%, 20%, 30%, 50%, and 80% were tested. The axial temperature drop along the length of the pipe was measured using K-type therm...

Loop heat pipe (LHP) development by utilizing coherent porous silicon (CPS) wicks

This paper introduces a theoretical model for a Loop Heat Pipe (LHP) utilizing a coherent porous silicon (CPS) wick. The paper investigates the effects of different parameters on the performance of the LHP such as evaporator temperature, condenser temperature, total mass charge, wick thickness, porosity, and pore size. A LHP is a two-phase device with extremely high effective thermal conductivity that uses capillary forces developed inside its wicked evaporator to pump a working fluid through a closed loop. The loop heat pipe is developed to efficiently transport heat that is generated by a highly localized concentrated heat source and then to discharge this heat to a convenient sink. This device is urgently needed to cool electronic components, especially in space applications. The LHP has been modeled utilizing the conservation equations and thermodynamic cycle. The loop heat pipe cycle is presented on a T-s diagram. A direct relation is developed between the ratio of heat going for evaporation as well as heat leaking to the compensation chamber.

MEMS loop heat pipe based on coherent porous silicon technology

This paper discusses the theory, modeling, design, fabrication and preliminary test results of the MEMS loop heat pipe being developed at the Center for Microelectronic Sensors and MEMS at the University of Cincinnati. The emphasis is placed upon the silicon micro wick and its production through a novel technique known as Coherent Porous Silicon (CPS) Technology.

Steady state model of a loop heat pipe (LHP) with coherent porous silicon (CPS) wick in the evaporator

A theoretical study is conducted to explore the effect of different parameters on the performance of a loop heat pipe (LHP). These parameters are evaporator temperature, condenser temperature, total mass charge, the tube size of the piping system, and pumping distance between evaporator and condenser. This paper presents a steady state model that describes the thermodynamics, heat transfer, and fluid mechanics inside an LHP. An LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference that developed between the evaporator and condenser to circulate a working fluid through a closed loop. The loop heat pipe efficiently transports the heat generated by a highly localized concentrated heat source and discharges this heat to a convenient sink. The steady state LHP model is described by the conservation equations, thermodynamic relations, and capillary and nucleate boiling limits. The loop heat pipe cycle is presented on a temperature-entropy diagram. A relationship is developed to predict the ratio of the heat of evaporation to the heat leaked to the compensation chamber. This work predicts the size of an LHP, the pumping distance, and the maximum power that can be dissipated for a fixed source temperature.

Test Cell for a Novel Planar MEMS Loop Heat Pipe Based on Coherent Porous Silicon

Work towards the development of an innovative, potentially high power density, MEMS loop heat pipe is in progress at the Center for Microelectronic Sensors and M E M S at the University of Cincinnati. The design of the loop heat pipe is based upon the very unique coherent porous silicon technology, a technique in which vast arrays of micrometer ‐ sized through ‐ holes are photo ‐ electrochemically etched into a silicon wafer perpendicular to the (100) surface. The initial mathematical model, the design, fabrication and characterization of the device in the open loop configuration were previously reported at this conference, STAIF 2002. This paper begins with a very brief explanation of the device and its theory of operation. The design of the device components and their production utilizing the various techniques of microelectronic and microelectromechanical fabrication are presented. The modifications made to the photon ‐ induced, electrochemical etch process, which significantly increase the etch rate o...

EXPERIMENTAL INVESTIGATION OF MICRO/NANO HEAT PIPE WICK STRUCTURES

The performance of electronic devices is limited by the capability to remove heat from these devices. A heat pipe is a device to facilitate heat transport that has seen increased usage to address this challenge. A heat pipe is a two-phase heat transfer device capable of transporting heat with minimal temperature gradient. An important component of a heat pipe is the wick structure, which transports the condensate from the condenser to the evaporator. The requirements for high heat transport capability and high resilience to external accelerations leads to the necessity of a design trade off in the wick geometry. This makes the wick performance a critical parameter in the design of heat pipes. The present study investigates experimental methods of testing capillary performance of wick structures ranging from micro- to nano-scales. These techniques will facilitate a pathway to the development of nano-engineered wick structures for high performance heat pipes.Copyright

The MEMS Loop Heat Pipe Based on Coherent Porous Silicon — The Modified System Test Structure

The previous papers presented at STAIF 2002 and STAIF 2003 discussed the design, fabrication and characterization of the evaporator section and the initial test cell of a planar MEMS loop heat pipe based upon coherent porous silicon or “CPS” technology. The potentially revolutionary advantage of CPS technology is that it is planar and allows for pores or capillaries of absolutely uniform diameter. Coherent porous silicon can be mass‐produced by various MEMS fabrication techniques. The preliminary experiments made with the original test structure exhibited the desired temperature and pressure differences, but these differences were extremely small and oscillatory. This paper describes modifications made to the initial test cell design, which were intended to improve its evacuated, closed loop performance. Included among these changes were the redesign of the compensation chamber and condenser, an increase in the porosity of the coherent porous silicon wick, the fabrication of silicon top “hot” plates with ...