Results for high heat-flux flow realizations in innovative operations of milli-meter scale condensers and boilers

Abstract

This paper defines the nature of high heat-flux innovative boiler and condenser operations that keeps these devices functional for many critically needed thermal management applications where gravitational force is negligible (naturally or made to be so) relative to other forces (shear, pressure, etc.). These applications include heat removal from narrow spaces to address electronic cooling needs (i.e. mm-scale boilers and condensers with μm-scale flows), gravity insensitive aircraft operations, and zero-gravity applications. Further, the paper presents fundamental condensing and boiling flow results that describe the physics that needs to be utilized to achieve significantly enhanced heat-flux values within the context of these applications.Reported experimental results demonstrate: (i) the ability to replace ineffective and problematic liquid–vapor configurations (plug-slug, etc.) with annular flows where a flowing thin liquid film covers the entire heat-exchange surface, (ii) the ability to use pulsation-induced interfacial wave phenomena in conjunction with sub-micron thin film phenomena (such as contact line physics) to achieve very high heat-flux during boiling and condensation, and (iii) the ability to desensitize these flows that would otherwise be highly sensitive to inadvertent but ever present flow fluctuations (due to sensitive coupling between vapor and liquid motions, as well as sensitivity of these micro-meter scale thick liquid films to minuscule transverse vibrations of the heat-exchange surface) when flow channel hydraulic diameters reduce to milli-meter or sub milli-meter scales.Innovative realizations of shear-driven boiling and condensing flows have been investigated within horizontal channels (with condensation or boiling on the bottom, horizontal surface). Results from boiling and condensing flows of FC-72 fluid in horizontal rectangular cross-section (2 or 6mm gap height and 15 or 24mm width, respectively) ducts of 1m length are presented. Utilizing a controlled presence of pulsatile mass flow rates, significant enhancements in heat-transfer rates are obtained for these innovative devices at a location within the device length – these enhancements (relative to non-pulsatile conditions) are sometimes >860% for condensing flows and >190% for boiling flows. The paper reports representative time-varying heat-flux values at this location in support of the understanding that this phenomena can potentially be used (in future experiments) to significantly enhance average heat-flux value over the entire length of an innovative boiler/condenser. The reported phenomena arises from superposing relatively fast time-scale (2.5–30Hz) flow rate pulsations on otherwise steady-in-the-mean flows to beneficially change certain flow variable averages – over the longer time-scale (<0.01Hz) of practical interest – relative to their values obtained in the absence of externally imposed pulsations. Such externally imposed pulsations may cause standing waves to form on the thin film interface. The observed asymmetric reduction in the mean film thickness (associated with high heat-flux values) is likely due to “stickiness” of wave-troughs resulting from interaction of phenomena associated with several length scales (namely nano-scale, micro-scale, and continuum length scales) and two different time scales – short (less than 0.5s) and long (greater than 1min). That is, associated micro-scale flows at wave-troughs interact with and, possibly, de-stabilize the local adsorbed layer (whose thickness may be <200nm and is exposed to phenomena such as disjoining pressures) on the wetting heat-exchange surface.