Jamie S. Ervin

Micrometer-Sized Water Droplet Impingement Dynamics and Evaporation on a Flat Dry Surface

A comprehensive numerical and experimental investigation on micrometer-sized water droplet impact dynamics and evaporation on an unheated, flat, dry surface is conducted from the standpoint of spray-cooling technology. The axisymmetric time-dependent governing equations of continuity, momentum, energy, and species are solved. Surface tension, wall adhesion effect, gravitational body force, contact line dynamics, and evaporation are accounted for in the governing equations. The explicit volume of fluid (VOF) model with dynamic meshing and variable-time stepping in serial and parallel processors is used to capture the time-dependent liquid-gas interface motion throughout the computational domain. The numerical model includes temperature- and species-dependent thermodynamic and transport properties. The contact line dynamics and the evaporation rate are predicted using Blakes and Schrages molecular kinetic models, respectively. An extensive grid independence study was conducted. Droplet impingement and evaporation data are acquired with a standard dispensing/imaging system and high-speed photography. The numerical results are compared with measurements reported in the literature for millimeter-size droplets and with current microdroplet experiments in terms of instantaneous droplet shape and temporal spread (R/D(0) or R/R(E)), flatness ratio (H/D(0)), and height (H/H(E)) profiles, as well as temporal volume (inverted A) profile. The Weber numbers (We) for impinging droplets vary from 1.4 to 35.2 at nearly constant Ohnesorge number (Oh) of approximately 0.025-0.029. Both numerical and experimental results show that there is air bubble entrapment due to impingement. Numerical results indicate that Blakes formulation provides better results than the static (SCA) and dynamic contact angle (DCA) approach in terms of temporal evolution of R/D(0) and H/D(0) (especially at the initial stages of spreading) and equilibrium flatness ratio (H(E)/D(0)). Blakes contact line dynamics is dependent on the wetting parameter (K(W)). Both numerical and experimental results suggest that at 4.5 < We < 11.0 the short-time dynamics of microdroplet impingement corresponds to a transition regime between two different spreading regimes (i.e., for We < or = 4.5, impingement is followed by spreading, then contact line pinning and then inertial oscillations, and for We > or = 11.0, impingement is followed by spreading, then recoiling, then contact line pinning and then inertial oscillations). Droplet evaporation can be satisfactorily modeled using the Schrage model, since it predicts both well-defined transient and quasi-steady evaporation stages. The model compares well with measurements in terms of flatness ratio (H/H(E)) before depinning occurs. Toroidal vortices are formed on the droplet surface in the gaseous phase due to buoyancy-induced Rayleigh-Taylor instability that enhances convection.

Evaporation Characteristics of Pinned Water Microdroplets

Alejandro M. Briones∗ and Jamie S. Ervin University of Dayton Research Institute, Dayton, Ohio 45469 Larry W. Byrd U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 Shawn A. Putnam Universal Technology Corporation, Dayton, Ohio 45434 Ashley White University of Dayton, Dayton, Ohio 45469 and John G. Jones∗∗ U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433

Refrigerant Charge Management and Control for Next-Generation Aircraft Vapor Compression Systems

Abstract : Vapor compression systems (VCS) offer significant benefits as the backbone for next generation aircraft thermal management systems (TMS). For a comparable lift, VCS offer higher system efficiencies, improved load temperature control, and lower transport losses than conventional air cycle systems. However, broad proliferation of VCS for many aircraft applications has been limited primarily due to maintenance and reliability concerns. In an attempt to address these and other VCS system control issues, the Air Force Research Laboratory has established a Vapor Cycle System Research Facility (VCSRF) to explore the practical application of dynamic VCS control methods for next-generation, military aircraft TMS. The total refrigerant mass contained within the closed refrigeration system (refrigerant charge) is a critical parameter to VCS operational readiness. Too much or too little refrigerant can be detrimental to system performance. Extreme values of refrigerant charge can lead to a loss of evaporator temperature control, loss of high side pressure control, or other potentially catastrophic occurrences. The objective of this work is to examine real-time methods for determination of acceptable refrigerant charge in a prototypical VCS system, as a function of operational points, using only sensors already utilized in the control system (in situ control sensors). It is envisioned that studies such as these can be used to guide development of a simple in situ prognostic tool for system state-of-health indication (i.e., ?Red Light, Yellow Light, Green Light?), with respect to level of charge, and to enable on-demand maintenance. Additionally, a method for continuous management of refrigerant charge as a means for optimizing system efficiency over a range of dynamic operating points is presented.

An Integrated Chemical Reactor-heat Exchanger based on Ammonium Carbamate

Abstract : In this work we present our recent effort in developing a novel heat exchanger based on endothermic chemical reaction (HEX reactor). The proposed HEX reactor is designed to provide additional heat sink capability for aircraft thermal management systems. Ammonium carbamate (AC) which has a decomposition enthalpy of 1.8 MJ/kg is suspended in propylene glycol and used as the heat exchanger working fluid. The decomposition temperature of AC is pressure dependent (60 ?C at 1 atmosphere; lower temperatures at lower pressures) and as the heat load on the HEX increases and the glycol temperature reaches AC decomposition temperature, AC decomposes and isothermally absorbs energy from the glycol. The reaction, and therefore the heat transfer rate, is controlled by regulating the pressure within the reactor side of the heat exchanger. The experiment is designed to demonstrate continuous replenishment of AC. This requires recovering the depleted glycol while expelling waste gases, dispersing and suspending fresh AC, and injecting the mixture into the heat exchanger. A gasketed plate heat exchanger is used as the reactor for this experiment, and heated water is used to provide the thermal load. The performance of the HEX reactor is characterized as a function of water flow rate and temperature, AC/glycol mixture flow rate, and AC concentration. Varying these parameters permits mapping the performance of the system under different conditions.

In-situ Charge Determination for Vapor Cycle Systems in Aircraft

Abstract : The Air Force Research Laboratory (AFRL), in cooperation with the University of Dayton Research Institute (UDRI) and Fairchild Controls Corporation, is operating an in-house advanced vapor compression refrigeration cycle system (VCS) test rig known as ToTEMS (Two-Phase Thermal Energy Management System). This test rig is dedicated to the study and development of VCS control and operation in support of the Energy Optimized Aircraft (EOA) initiative and the Integrated Vehicle ENergy Technology (INVENT) program. Previous papers on ToTEMS have discussed the hardware setup and some of the preliminary data collected from the system, as well as the first steps towards developing an optimum-seeking control scheme. A key goal of the ToTEMS program is to reduce the risk associated with operating VCS in the dynamic aircraft environment. One of the key questions regarding the operability of VCS in aircraft which will be addressed is the in-situ measurement of refrigerant charge within the VCS system. Several potential methods of determining whether an appropriate charge of refrigerant exists will be discussed. An appropriate charge level is one which enables safe and efficient operation of the VCS over its designed operating envelop.

Thin Film Evaporation Model With Retarded van der Waals Interaction

Abstract : In phase change heat transfer equipment, three-phase contact regions exist that consist of a solid wall and the liquid and vapor phases of a working fluid. When the working fluid fully wets the solid wall, a microscopic thin film adjoining the meniscus is present called the adsorbed film. Upon heating, a non-uniform evaporative flux profile develops with a maximum value occurring within the transition between the adsorbed film and the intrinsic meniscus. It is important to study the heat transfer characteristics of this region to gain better fundamental understanding and useful design principles. The adsorbed film occurs when the driving potential for evaporation is opposed by the presence of intermolecular forces, represented analytically by the disjoining pressure, which acts to thicken a wetting film. The model presented includes lubrication theory of the liquid flow within the film, heat conduction across the film from the heated wall to the liquid-vapor interface, kinetic theory evaporation from the interface to the vapor phase, and disjoining pressure based on a retarded van der Waals interaction. The retarded van der Waals interaction is derived from Hamaker theory, the summation of retarded pair potentials for all molecules for a given geometry. When combined, the governing equations form a third-order, nonlinear differential equation for the film thickness versus distance, which is solved numerically using iteration of the initial film curvature in order to match the far-field curvature of the meniscus. Also, iteration is required at each length step to determine the liquid-vapor interface temperature. Useful outputs of the model include the liquid-vapor interface temperature and the evaporative mass flux profile. The model is calibrated to in-house experiments that employ an axisymmetric capillary feeder to provide a thin film of n-octane onto a substrate of silicon, where the gas phase is air saturated with vapor. The film thickness versus radial distance

Heat Transfer Coefficients and Lifetimes of Micro-Droplet Evaporation in the Transition Regime

The goal of the present work is to determine the heat transfer characteristics of evaporating micro-droplets of water from a hot surface. To accomplish this, a one-dimensional, finite-difference model is used to simulate the transport of water vapor and energy from the droplet’s liquid-vapor interface toward and inside the hemispherical, gaseous region surrounding the droplet. The model incorporates a transition regime correction to the kinetic theory evaporative mass flux. The transition regime correction, a multiplier applied to the kinetic flux, is a function of the Knudsen number, the ratio of molecular mean free path to the droplet radius. The transition regime encompasses droplet sizes for which neither the kinetic model of evaporation nor the hydrodynamic continuum theory is entirely appropriate. The model simulates the liquid phase as one-dimensional conduction between the hot surface and the liquid-vapor interface. Previously, we validated our model against measured volume data as a function of time for several evaporating droplets. Using the model, overall heat transfer coefficients and total evaporation times are determined. Linear fits of both are provided against dimensional groupings of initial droplet volume and surface temperature superheat.Copyright

Facilities, Instrumentation, and Modeling for Fluid Filled Tube Experimentation

There has long been interest in the cooling of gas turbines by liquids rather than air due to the relatively higher convective heat transfer rates available with liquids. Early experimental studies which used water were subject to leaking seals, fouling, corrosion and other problems. This paper describes newly created facilities for the study of the flow of a hydrocarbon fuel through a heated, rotating passage. Since hydrocarbon fuels possess a range of thermal stability behavior, different fuel types will be selected for study. Buoyancy-dominated velocity fields under high acceleration fields are known for having complex behavior. Fluid dynamics simulations permit visualization and analysis of the flow for behavior that is otherwise unobservable in a metal tube. In this research, the unsteady three-dimensional Navier-Stokes and energy equations are solved. There is little available in the literature concerning the study of heated, rotating hydrocarbon liquids. The goal of this experimental and computational research is to study the influence of rotation on the resulting heat transfer and flow for a heated, rotating hydrocarbon liquid.