Tim Ameel

Laminar fluid behavior in microchannels using micropolar fluid theory

Abstract In this paper, we describe microchannel fluid behavior using a numerical model based on micropolar fluid theory and experimentally verify the model using micromachined channels. The micropolar fluid theory augments the laws of classical continuum mechanics by incorporating the effects of fluid molecules on the continuum. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 50 to 600 μm and heights ranging from 20 to 30 μm. A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record pressure drop. The experimental data obtained for water showed an increase in the Darcy friction factor when compared to traditional macroscale theory, especially at the lower Reynolds number flows. The numerical model of the micropolar fluid theory predicted experimental data better than the classical Navier–Stokes theory and the model compares favorably with the currently available experimental data.

Slip-flow heat transfer in rectangular microchannels

Abstract Laminar slip-flow forced convection in rectangular microchannels is studied analytically by applying a modified generalized integral transform technique to solve the energy equation, assuming hydrodynamically fully developed flow. Results are given in terms of the fluid mixed mean temperature, and both local and fully developed mean Nusselt numbers. Heat transfer is found to increase, decrease, or remain unchanged, compared to non-slip-flow conditions, depending on two dimensionless variables that include effects of rarefaction and the fluid/wall interaction. The transition point at which the switch from heat transfer enhancement to reduction occurs is identified for different aspect ratios.

The Graetz problem extended to slip-flow

Abstract The original problem for thermally developing heat transfer in laminar flow through a circular tube, as formulated by Graetz, did not consider the ‘slip-flow’ condition. This paper extends the original work of Graetz to include the effect of slip-flow, which occurs in gases at low pressures or in microtubes at ordinary pressures. A special technique was developed to evaluate the eigenvalues for the problem. Eigenvalues were evaluated for Knudsen numbers ranging between 0 and 0.12. Simplified relationships were developed to describe the effect of slip-flow on the convection heat transfer coefficient.

Miniaturization technologies applied to energy systems

An overview of the miniaturization technologies and their application to energy systems is presented. Based on the technologies referred to as MEMS (microelectromechanical systems) or MST (micro systems technologies), silicon based micromachining, deep X-ray lithography and the micro mechanical machining processes (micro drilling, milling, cutting, electrical discharge machining, laser, focused ion beams, etc.) will be discussed in the context of application to fluid flow, heat transfer, fluidics and energy systems. An overview of fundamental research and applications will be made with emphasis on the work in the United States. The collaborative work in progress by the Institute for Micromanufacturing and the Pacific Northwest National Laboratories is highlighted. In particular, the on-going development of a micro HVAC (heating, ventilating and air conditioning) system is given specific attention. Devices and/or systems such as micro heat pumps, heat pipes, evaporators, condensers, heat exchangers, compressors and the like will be presented. Advantages, disadvantages and the rationale for miniaturization will be discussed. Current needs and markets will be discussed along with a discussion for future needs.

Evaluation of the eigenvalues for the graetz problem in slip-flow

Abstract A technique was developed for evaluation of the eigenvalues for the Graetz problem extended to slip-flow. The first four eigenvalues for Knudsen numbers of 0.02, 0.04, …, 0.12 were found. By using a least square curve-fit method, simplified relationships between the eigenvalues and Knudsen number were obtained. The efficient and accurate determination of the eigenvalues will lead to better predictions of heat transfer in rarefied gas flows and for gas flows in microtubes.

Performance predictions of alternative, low cost absorbents for open-cycle absorption solar cooling

To achieve solar fractions greater than 0.90 using the open-cycle absorption refrigeration system, considerable sorbent solution storage is necessary. Sorbent solutions currently under consideration, such as aqueous solutions of lithium chloride and lithium bromide, may be too costly to exploit the open-cycle storage concept. Having identified the absorber as the system component whose performance is affected the most by a change in absorbent, an absorber model was selected from available literature pertaining to simultaneous heat and mass transfer. Low cost absorbent candidates were selected and their physical properties were either located in the literature, measured, or estimated. Absorber operating parameters were selected and the model was then used to estimate absorber performance for each absorbent in terms of cooling capacity per unit of absorber area. After specifying system parameters such as absorber capacity and cooling load, the absorber area, absorbent cost, and sorbent solution pumping power and storage volume were estimated for each candidate. The most promising of the absorbents considered was a mixture of two parts lithium chloride and one part zinc chloride. The estimated capacities per unit absorber area were 50–70% less than those of lithium bromide; however, the lithium bromide cost for a system sized to cool a 190 m2 residential structure was estimated to be eight times that for the lithium-zinc chloride mixture. Both the lithium-zinc chloride mixture and lithium bromide solutions had estimated pumping powers of less than 0.1 kW. The solubility of the lithium-zinc chloride mixture at absorber conditions was improved over that of lithium bromide, reducing the risk of solidification of the solution.

Effects of rectangular microchannel aspect ratio on laminar friction constant

In this paper, the effects of rectangular microchannel aspect ratio on laminar friction constant are described. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 150 micrometers to 600 micrometers and heights ranging from 22.71 micrometers to 26.35 micrometers . A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record the pressure drop. The experimental data obtained for water for flows at Reynolds numbers below 10 showed an approximate 20% increase in the friction constant for a specified driving potential when compared to macroscale predictions from the classical Navier-Stokes theory. When the experimental data are studied as a function of aspect ratio, a 20% increase in the friction constant is evident at low aspect ratios. A similar increase is shown by the currently available experimental data for low Reynolds number (< 100) flows of water.

Slip flow convection in isoflux rectangular microchannels

Laminar forced convection in thermally developing slip flow through isoflux rectangular microchannels is analytically investigated. Local and fully developed Nusselt numbers, fluid temperatures, and wall temperatures are obtained by solving the continuum energy equation for hydrodynamically fully developed slip flow with the velocity slip and temperature jump condition at the walls. Heat transfer may increase, decrease, or remain unchanged, compared to nonslip flow conditions, depending on aspect ratios and two-dimensionless variables that include effects of the microchannel size or rarefaction and the fluid/wall interaction. The transition points that separate heat transfer enhancement from reduction are acquired for different aspect ratios

Experimental Studies of Natural Convection in Partitioned Enclosures With a Trombe Wall Geometry

Natural convection heat transfer was investigated in a scaled test facility of a Trombe wall geometry. A silicone oil was employed as the convecting medium to obtain large Rayleigh numbers (up to 1.5 {times} 10{sup 10}, based on enclosure height) characteristic of a full-scale Trombe wall in a passive solar building. The main objectives were to study the effects of Trombe wall nonisothermality and location on heat transfer, fluid temperature and fluid flow patterns. As expected, Nusselt numbers were slightly larger on the Trombe wall space side than on the living space side. Nusselt numbers increased slightly as the mass transfer gaps in the Trombe wall were increased. The results were verified, for the zero gap case, by comparing with previous studies. Physical understanding of the convection process was enhanced by flow visualization data. The information obtained should be useful to designers in optimizing overall building performance for passive solar heating.

Friction and convection studies of R-134a in microchannels within the transition and turbulent flow regimes

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.