Design and fabrication of a microscale Joule -Thomson refrigerator

A simple thermodynamic, heat transfer, and fluid flow model was developed for microscale Joule-Thomson refrigerators (JT devices). For a given geometry, the model predicted that the cooling capacity of the refrigerator increased with the inlet refrigerant pressure. The effectiveness of the JT device also increased with the inlet pressure, and the heat exchanger channel length. At a constant inlet pressure, the effectiveness, and the refrigeration capacity of a given JT device increased as the aspect ratio of heat exchanger channels was increased. For nitrogen refrigerant, the model predicted that it was possible to obtain approximately 250 mW of refrigeration capacity at 82 K with 10 MPa (100 atm) of inlet pressure and a flow rate of 15.17 ml/s at standard pressure and temperature (STP). This prediction was justified by experimental values of Little (1984) who obtained 250 mW of refrigeration capacity at 83 K with 10 MPa (100 atm) of inlet pressure and a flow rate of 18 ml/s at STP. The simulation model was also used to design a novel JT device based on a layered arrangement of the evaporator, capillary, and the heat exchanger. The proposed JT device would have produced approximately 250 mW of refrigeration capacity at 100 K, for an inlet pressure of 6 MPa (60 atm). This proposed JT device was fabricated on silicon wafers using photolithography. The heat exchanger channels had a cross section of 50 x 20 μm and a length of 6 cm. The capillary channel cross section was 20 x 20 μm and its length was 6 cm. Both the length and the width of the evaporator was 30 mm, and its depth was 20 μm. Pyrex 7740 glass wafers (3 mm thick) were used to separate the evaporator from the capillary and the capillary from the heat exchanger. The heat exchanger was bonded with a top glass cover plate. Most layers were successfully bonded using the anodic bonding procedure. After bonding the evaporator to a glass wafer, subsequent anodic bonding was carried out by applying voltage from sides of each glass and silicon wafer. This bonding attempt demonstrated that the anodic bonding procedure could be used in packaging several silicon and glass wafers. The packaged device held together briefly but later separated due to poor bonding quality of the capillary and the heat exchanger. This poor bonding quality may have resulted from inadequate surface quality of silicon wafers. However, the knowledge and the experience gained in this work will be very useful in future development of JT devices

Thermal management of the hotspots in 3-D integrated circuits

Vertical integration for microelectronics possesses significant challenges due to its fast dissipation of heat generated in multiple device planes. This paper focuses on thermal management of a 3-D integrated circuit, and micro-channel cooling is adopted to deal with the 3-D integrated circuitthermal problems. In addition, thermal through-silicon vias are also used to improve the capacity of heat trans-mission. It is found that combination of microchannel cooling and thermal through-silicon vias can remarkably alleviate the hotspots. The results presented in this paper are expected to aid in the development of thermal design guidelines for 3-D integrated circuits

Numerical Simulation of Ice Slurry Flow in improved Plate Heat Exchanger Geometries with Consideration of different Phase Interaction Parameters

Concerning the cold supply, ice slurry technology is a safe, environmentally friendly and efficient solution for energy storage. Previous studies have shown that ice slurry is potentially one of the most important phase change material (PCM) slurries used as a secondary refrigerant due to its high cooling capacity and flexibility in application. However, agglomeration phenomena are observed with ice particle slurry flows, which can lead to blocking of e.g. heat exchangers and pipes. The study applies an Euler-Euler approach based on the kinetic theory of granular flow to describe the melting of an ice slurry flow. The Eulerian model was used to simulate the ice slurry in new channel structures of specially manufactured plate heat exchangers. The mass flow rate, ice concentration, and industrial ice particle sizes were varied. The validation with experimental data is presented. The results show the pressure drop and agglomeration reduction with the new channel design relative to commercially available geometries of the plate heat exchanger. The simulation results show the velocity profiles, the volume fraction, and the pressure loss related to the ice concentration. The simulation results of improved heat exchanger plates to avoid high volume fractions at the inlet and outlet as well as the channels of specially designed plate heat exchanger are displayed. The results show the effect of changing the phase interaction parameter at high velocity

Assessment of one equation model for natural convection and the prediction of impingement heat transfer

Spallart and Allmaras is a one-equation turbulence model developed for predicting complex aerodynamic flows. It will be of great interest to extend its application to predict other kind of flows in presence of heat and mass transfer. Specifically we will demonstrate, by considering two applications, that this one equation model may replace the two equation models in several heat transfer applications such as HVAC, electronics cooling and fire propagation in buildings. This will reduce computing time and numerical stiffness in CFD calculations. The first application is the 2D natural turbulent convection in a cavity differentially heated; which was considered as a benchmark case before applying CFD in any HVAC configuration. The exact configuration proposed by Ampofo, was then modeled and the radiation effect was studied. The second application proposed by Riera was the numerical prediction of a heat exchanger based on hybrid jet impingement/micro-channel; it was used for managing high heat-flux thermal of power electronics devices. Those applications deal with low and high fluxes transfer respectively and allow demonstrating clearly the capacity of the one equation model to predict confined turbulent natural/ forced convection and jet-impingement heat transfer.This project was funded by the deanship of Scientific Research (DSR) at king Abdulaziz University, Jeddah, under Grant no: G-568-305-1436

Lipid ion channels and the role of proteins

Synthetic lipid membranes in the absence of proteins can display quantized conduction events for ions that are virtually indistinguishable from those of protein channel. By indistinguishable we mean that one cannot decide based on the current trace alone whether conductance events originate from a membrane, which does or does not contain channel proteins. Additional evidence is required to distinguish between the two cases, and it is not always certain that such evidence can be provided. The phenomenological similarities are striking and span a wide range of phenomena: The typical conductances are of equal order and both lifetime distributions and current histograms are similar. One finds conduction bursts, flickering, and multistep-conductance. Lipid channels can be gated by voltage, and can be blocked by drugs. They respond to changes in lateral membrane tension and temperature. Thus, they behave like voltage-gated, temperature-gated and mechano-sensitive protein channels, or like receptors. Lipid channels are remarkably under-appreciated. However, the similarity between lipid and protein channels poses an eminent problem for the interpretation of protein channel data. For instance, the Hodgkin-Huxley theory for nerve pulse conduction requires a selective mechanism for the conduction of sodium and potassium ions. To this end, the lipid membrane must act both as a capacitor and as an insulator. Non-selective ion conductance by mechanisms other than the gated protein-channels challenges the proposed mechanism for pulse propagation. ... Some important questions arise: Are lipid and protein channels similar due a common mechanism, or are these similarities fortuitous? Is it possible that both phenomena are different aspects of the same phenomenon? Are lipid and protein channels different at all? ... (abbreviated)Comment: 10 pages, 10 figures - accepted by 'Accounts of Chemical Research

Lipid Ion Channels

The interpretation electrical phenomena in biomembranes is usually based on the assumption that the experimentally found discrete ion conduction events are due to a particular class of proteins called ion channels while the lipid membrane is considered being an inert electrical insulator. The particular protein structure is thought to be related to ion specificity, specific recognition of drugs by receptors and to macroscopic phenomena as nerve pulse propagation. However, lipid membranes in their chain melting regime are known to be highly permeable to ions, water and small molecules, and are therefore not always inert. In voltage-clamp experiments one finds quantized conduction events through protein-free membranes in their melting regime similar to or even undistinguishable from those attributed to proteins. This constitutes a conceptual problem for the interpretation of electrophysiological data obtained from biological membrane preparations. Here, we review the experimental evidence for lipid ion channels, their properties and the physical chemistry underlying their creation. We introduce into the thermodynamic theory of membrane fluctuations from which the lipid channels originate. Furthermore, we demonstrate how the appearance of lipid channels can be influenced by the alteration of the thermodynamic variables (temperature, pressure, tension, chemical potentials) in a coherent description that is free of parameters. This description leads to pores that display dwell times closely coupled to the fluctuation lifetime via the fluctuation-dissipation theorem. Drugs as anesthetics and neurotransmitters are shown to influence the channel likelihood and their lifetimes in a predictable manner. We also discuss the role of proteins in influencing the likelihood of lipid channel formation.Comment: Revie

Changes in single K+ channel behavior through the lipid phase transition

We show that the activity of an ion channel is strictly related to the phase state of the lipid bilayer hosting the channel. By measuring unitary conductance, dwell times, and open probability of the K+ channel KcsA as a function of temperature in lipid bilayers composed of POPE and POPG in different relative proportions, we obtain that all those properties show a trend inversion when the bilayer is in the transition region between the liquid disordered and the solid ordered phase. These data suggest that the physical properties of the lipid bilayer influence ion channel activity likely via a fine tuning of its conformations. In a more general interpretative framework, we suggest that other parameters such as pH, ionic strength, and the action of amphiphilic drugs can affect the physical behavior of the lipid bilayer in a fashion similar to temperature changes resulting in functional changes of transmembrane proteins

Numerical investigation of the energy performance of a guideless irregular heat and mass exchanger with corrugated heat transfer surface for dew point cooling

© 2016 The Author(s) The paper presents an investigation into the energy performance of a novel irregular heat and mass exchanger for dew point cooling which, compared to the existing flat-plate heat exchangers, removed the use of the channel supporting guides and implemented the corrugated heat transfer surface, thus expecting to achieve the reduced air flow resistance, increased heat transfer area, and improved energy efficiency (i.e. Coefficient of Performance (COP)) of the air cooling process. CFD simulation was carried out to determine the flow resistance (K) factors of various elements within the dry and wet channels of the exchanger, while the ‘finite-element’ based ‘Newton-iteration’ numerical simulation was undertaken to investigate its cooling capacity, cooling effectiveness and COP at various geometrical and operational conditions. Compared to the existing flat-plate heat and mass exchangers with the same geometrical dimensions and operational conditions, the new irregular exchanger could achieve 32.9%–37% higher cooling capacity, dew-point and wet-bulb effectiveness, 29.7%–33.3% higher COP, and 55.8%–56.2% lower pressure drop. While undertaking dew point air cooling, the irregular heat and mass exchanger had the optimum air velocity of 1 m/s within the flow channels and working-to-intake air ratio of 0.3, which allowed the highest cooling capacity and COP to be achieved. In terms of the exchanger dimensions, the optimum height of the channel was 5 mm while its length was in the range 1–2 m. Overall, the proposed irregular heat and mass exchanger could lead to significant enhanced energy performance compared to the existing flat-plate dew point cooling heat exchanger of the same geometrical dimensions. To achieve the same amount cooling output, the irregular heat and mass exchanger had the reduced size and cost against the flat-plate ones

Monogroove heat pipe design: Insulated liquid channel with bridging wick

A screen mesh artery supported concentrically within the evaporator section of a heat pipe liquid channel retains liquid in the channel. Continued and uniform liquid feed to the heat pipe evaporation section (20) during periods of excessive heat transfer is assured. The overall design provides high evaporation and condensation film coefficients for the working fluid by means of the circumferential grooves in the walls of the vapor channel, while not interfering with the overall heat transport capability of the axial groove. The design has particular utility in zero-g environments