Surprising heat transfer regimes have been discovered with a micron-sized heat transfer element operated in subcooled (20 degrees C) demineralized water over a wide pressure range (0 - 400 atm) at heat fluxes up to 4000 W/cm2. The platinum heat transfer element (7.5 diameter microns, length 1 mm) is installed within a one cm3 stainless steel chamber. Sealed electrical terminals penetrate the chamber to effect direct current heating of the platinum element. The pressure is established with a pneumatic pump. The platinum element has an adiabatic heating rate of one degree C per microsecond at 3700 W/cm2, therefore response is essentially instantaneous for the procedure described herein. The direct current voltage and current are measured from which the power to and the resistance (temperature) of the platinum element are determined.
The discovery of the new heat transfer regimes was effected as follows: 1.) The water-filled stainless steel chamber was pressurized to 400 atm. 2.) Power was applied at 3000 W/cm2. 3.) Power was maintained substantially constant as pressure was smoothly reduced from 400 atm to 13 atm over a period of about 20 seconds. Voltage, amperage, and pressure were recorded at 0.1 second intervals as pressure was reduced.
The new heat transfer phenomena thus discovered are as follows: 1.) As pressure was reduced from 400 atm to 270 atm, the temperature of the platinum element smoothly increased from 370 degrees C to 380 degrees C. 2.) At 270 atm the temperature of the element abruptly stepped upward to 590 degrees C. (NEW) 3.) Temperature smoothly increased to 730 degrees C until pressure reached 220 atm. (NEW). 4.) In the vicinity of the critical pressure, the temperature of the element turned around and began smoothly decreasing. (NEW) 5.) At 160 atm, the temperature stepped down from 520 to 350 degrees C. (NEW) 6.) Temperature smoothly decreased to 230 degrees C as pressure reached 13 atm and power was turned off.
The high priority immediate application of these discoveries lies in the evaluation of nuclear reactor power excursions such as occurred at Chernobyl and also in Idaho during early 1961. In those cases, micron-sized particles were blasted into surrounding water where they transferred heat to subcooled water at ultra high heat fluxes and at pressers up to 600 atmospheres.
There are countless research paths that stem from these discoveries. For example, current density within the 7.5 micron platinum element may reach 106 W/ W/cm2. With copper, the current density would reach three times that. And with copper at a diameter of 0.75 microns the potential becomes 107 W/ W/cm2. Another example, others have constructed an array of microheaters on various substrates (easily achieved in comparison with a free standing wire). Applying those devices beyond the relatively low pressures and heat fluxes explored to date, would yield a vast array of discoveries and applications with several fluids. That is especially the case for the nanometer regime.
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