Terrence W. Simon

Measurements in Film Cooling Flows: Hole L/D and Turbulence Intensity Effects

Hot-wire anemometry measurements of simulated film cooling are presented to document the influence of the free-stream turbulence intensity and film cooling hole length-to-diameter ratio on mean velocity and on turbulence intensity. Measurements are taken in the zone where the coolant and free-stream flows mix. Flow from one row of film cooling holes with a streamwise injection of 35 deg and no lateral injection and with a coolant-to-free-stream flow velocity ratio of 1.0 is investigated under free-stream turbulence levels of 0.5 and 12 percent. The coolant-to-free-stream density ratio is unity. Two length-to-diameter ratios for the film cooling holes, 2.3 and 7.0, are tested. The measurements document that under low free-stream turbulence conditions pronounced differences exist in the flowfield between L/D = 7.0 and 2.3. The differences between L/D cases are less prominent at high free-stream turbulence intensities. Generally, short-L/D injection results in jetting of the coolant farther into the free-stream flow and enhanced mixing. Other changes in the flowfield attributable to a rise in free-stream turbulence intensity to engine-representative conditions are documented.

A technique for enhancing boiling heat transfer with application to cooling of electronic equipment

Particle layering is introduced as an effective and convenient technique for enhancing boiling nucleation on a surface. Because it can be applied without stress or damage to a surface, it can be implemented in immersion cooling, with boiling, of electronic equipment components. Such an enhanced surface, which has an increased number of nucleation sites, shows a decreased level of wall superheat under boiling and an increased critical heat flux relative to superheat and critical heat flux values for an untreated surface. Application of this technique results in a decrease of heated surface temperature and a more uniform temperature of the heated surface; both effects are important in immersion cooling of electronic equipment. >

Contact angle effects on boiling incipience of highly-wetting liquids

Abstract The difficulty in predicting the boiling incipience of highly-wetting liquids has slowed the application of immersion cooling technology in the electronics industry. The present effort sheds new light on this phenomenon by examining the influence of the dynamic solid/liquid contact angle and contact angle hysteresis on the incipience superheat. The results suggest that variations in contact angle, induced by changes in the direction and magnitude of the liquid/vapor interface velocity, can substantially affect the formation of bubble embryos and may well explain the wide experimental scatter in incipience superheat values reported for highly-wetting liquids.

wall Superheat Excursions in the Boiling incipience of Dielectric Fluids

Many of the candidate fluids for immersion cooling of microelectronic components possess both low surface tension and high gas solubility. As a consequence, ebullient heat transfer with such fluids is accompanied by nucleation anomalies and a frequently observed wall temperature overshoot. The difficulty in preventing this thermal excursion and in predicting its magnitude constrains the development of immersion cooling systems. This paper begins with a brief review of the mechanisms that may be responsible for delayed nucleation and examines the limited literature on incipience superheat excursions.

Experimental investigation of nucleate boiling incipience with a highly-wetting dielectric fluid (R-113)

Abstract Experiments on pool boiling heat transfer in saturated R-113 at 1 atm pressure are conducted to investigate anomalies associated with the initiation of boiling using horizontal, 0.13 mm diameter chromel wires and a 0.51 mm diameter, platinum thin-film heater. Wall incipience superheat values of up to 73°C and wide variations in incipience superheats (more than 20°C) from case to case for nominally identical cases are observed. Presentation of incipience superheat results in terms of probability distributions is therefore recommended. The significant influence of the minimum heat flux and corresponding minimum wall superheat (after the cool-down portion of one run and the beginning of the subsequent run) on the incipience superheat excursion is observed. Tests with large step changes in heat flux, using a thin-film. cylindrical heater, indicated increases in incipience superheat of as much as 20% over tests with a series of incremental steps in heat flux up to the same heat flux level.

Effects of dissolved gas content on pool boiling of a highly wetting fluid

Experimental results on pool boiling heat transfer from a horizontal cylinder in an electronic cooling fluid (FC-72) are presented. The effects on the boiling curve of having air dissolved in the fluid are documented, showing that fluid in the vicinity of the heating element is apparently liberated of dissolved gas during boiling. Dissolved gas was found to influence boiling incipience only with high gas concentrations (>0.005 moles/mole). For low-to-moderate concentrations, a larger superheat is required to initiate boiling and a hysteresis is observed between boiling curves taken with increasing and decreasing heat flux steps. Boiling, a very effective mode of heat transfer, is attractive for electronics cooling. The present experiment provides further documentation of the role of dissolved gas on the incipience process and shows similarities with subcooled boiling of a gas-free fluid. 20 refs., 8 figs., 1 tab.

Heat transfer - A review of 2001 literature

2. Conduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.1. Contact conduction and contact resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.2. Micro/nanoscale thermal effects, laser pulse heating, and hyperbolic heat transport . . 1892 2.3. Composites, heterogeneous media and complex geometries . . . . . . . . . . . . . . . . . . . 1893 2.4. Conduction with convection, phase change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 2.5. Analytical, numerical and experimental studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 2.6. Thermomechanical problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 2.7. Miscellaneous and special applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893

Turbine Endwall Aerodynamics and Heat Transfer

This review addresses recent literature on turbine passage aerodynamics and endwall heat transfer; articles that describe the endwall flow and cooling problems are summarized, recent activity on improving endwall aerothermal design is discussed, improved cooling schemes are proposed, and methods for managing secondary flows to allow more effective cooling are suggested. Much attention is given to aerodynamic losses associated with secondary flows developed near the endwalls. The endwall region flowfield is influenced by the stagnation zones established as the endwall approach flow boundary layer meets the airfoil leading edges, by the curvature of the passages, by the steps and gaps on the endwall surface ahead of and within the passage, by the leakage and coolant flows introduced through the endwall surface ahead of and within the passage, by the tip leakage flows between the blades and shroud in the rotor endwall region, and by many more effects. Recent combustor redesigns have flattened the turbine inlet temperature profile and have raised the turbine inlet temperatures. This, coupled with a continued need to improve engine durability and availability, has spurred strong interest in thermal control of the turbine endwall regions. Thus, much of the literature presented herein is focused on endwall cooling and, in particular, the effects of near-endwall secondary flows on endwall cooling.

Measurements in a Turbine Cascade Flow Under Ultra Low Reynolds Number Conditions

With the new generation of gas turbine engines, low Reynolds number flows have become increasingly important. Designers must properly account for transition from laminar to turbulent flow and separation of the flow from the suction surface, which is strongly dependent upon transition. Of interest to industry are Reynolds numbers based upon suction surface length and flow exit velocity below 150,000 and as low as 25,000. In this paper, the extreme low end of this Reynolds number range is documented by way of pressure distributions, loss coefficients, and identification of separation zones. Reynolds numbers of 25,000 and 50,000 and with 1 percent and 8-9 percent turbulence intensity of the approach flow (free-stream turbulence intensity, FSTI) were investigated. At 25,000 Reynolds number and low FSTI, the suction surface displayed a strong and steady separation region. Raising the turbulence intensity resulted in a very unsteady separation region of nearly the same size on the suction surface. Vortex generators were added to the suction surface, but they appeared to do very little at this Reynolds number. At the higher Reynolds number of 50,000, the low-FSTI case was strongly separated on the downstream portion of the suction surface. The separation zone was eliminated when the turbulence level was increased to 8-9 percent. Vortex generators were added to the suction surface of the low-FSTI case. In this instance, the vortices were able to provide the mixing needed to re-establish flow attachment. This paper shows that massive separation at very low Reynolds numbers (25,000) is persistent, in spite of elevated FSTI and added vortices. However, at a higher Reynolds number, there is opportunity for flow reattachment either with elevated free-stream turbulence or with added vortices. This may be the first documentation of flow behavior at such low Reynolds numbers. Although it is undesirable to operate under these conditions, it is important to know what to expect and how performance may be improved if such conditions are unavoidable.

Experimental Investigation of Transition to Turbulence as Affected by Passing Wakes

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section geometry is designed to simulate the effects of unsteady wakes resulting from rotor-stator interaction upon laminar-to-turbulent transition in turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire, thermal anemometry techniques are used to measure time-resolved and phase-averaged, wall-normal profiles of velocity, turbulence intensity and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 50,000. This study compares a previously documented base case flow having an approach flow turbulence intensity of 2.5 percent and a wake passing Strouhal number of 0.792 to two additional cases: one having an increased rod spacing case having a wake passing Strouhal number of 0.396, and another having an elevated approach flow turbulence intensity of 10 percent. From these data, the effects of increased rod spacing and elevated FSTI upon transition and separation processes in the near-wall flow are documented. The results show that a decreased wake passing Strouhal number results in an earlier separation with a larger separation bubble, while the elevated FSTI results in earlier separation, but with a shorter, thinner, separation bubble. The data and animations are included in an accompanying CD ROM.