Showing papers on "Critical heat flux published in 2012"

Structured surfaces for enhanced pool boiling heat transfer

Abstract: We experimentally investigated surface roughness-augmented wettability on critical heat flux (CHF) during pool boiling with horizontally oriented surfaces. Microstructured surfaces with a wide range of well-defined surface roughness were fabricated, and a maximum CHF of ∼208 W/cm2 was achieved with a surface roughness of ∼6. An analytical force-balance model was extended to explain the CHF enhancement. The excellent agreement found between the model and experimental data supports the idea that roughness-amplified capillary forces are responsible for the CHF enhancement on structured surfaces. The insights gained from this work suggest design guidelines for new surface technologies with high heat removal capability.

History, Advances, and Challenges in Liquid Flow and Flow Boiling Heat Transfer in Microchannels: A Critical Review

Abstract: As the scale of devices becomes small, thermal control and heat dissipation from these devices can be effectively accomplished through the implementation of microchannel passages. The small passages provide a high surface area to volume ratio that enables higher heat transfer rates. High performance microchannel heat exchangers are also attractive in applications where space and/or weight constraints dictate the size of a heat exchanger or where performance enhancement is desired. This survey article provides a historical perspective of the progress made in understanding the underlying mechanisms in single-phase liquid flow and two-phase flow boiling processes and their use in high heat flux removal applications. Future research directions for (i) further enhancing the single-phase heat transfer performance and (ii) enabling practical implementation of flow boiling in microchannel heat exchangers are outlined.

Enhancing Flow Boiling Heat Transfer in Microchannels for Thermal Management with Monolithically-Integrated Silicon Nanowires

Abstract: Thermal management has become a critical issue for high heat flux electronics and energy systems. Integrated two-phase microchannel liquid-cooling technology has been envisioned as a promising solution, but with great challenges in flow instability. In this work, silicon nanowires were synthesized in situ in parallel silicon microchannel arrays for the first time to suppress the flow instability and to augment flow boiling heat transfer. Significant enhancement in flow boiling heat transfer performance was demonstrated for the nanowire-coated microchannel heat sink, such as an early onset of nucleate boiling, a delayed onset of flow oscillation, suppressed oscillating amplitudes of temperature and pressure drop, and an increased heat transfer coefficient.

Gas Microflows in the Slip Flow Regime: A Critical Review on Convective Heat Transfer

Abstract: Accurate modeling of gas microvection is crucial for a lot of MEMS applications (microheat exchangers, pressure gauges, fluidic microactuators for active control of aerodynamic flows, mass flow and temperature microsensors, micropumps, and microsystems for mixing or separation for local gas analysis, mass spectrometers, vacuum, and dosing valves…). Gas flows in microsystems are often in the slip flow regime, characterized by a moderate rarefaction with a Knudsen number of the order of 10 � 2 ‐10 � 1 . In this regime, velocity slip and temperature jump at the walls play a major role in heat transfer. This paper presents a state of the art review on convective heat transfer in microchannels, focusing on rarefaction effects in the slip flow regime. Analytical and numerical models are compared for various microchannel geometries and heat transfer conditions (constant heat flux or constant wall temperature). The validity of simplifying assumptions is detailed and the role played by the kind of velocity slip and temperature jump boundary conditions is shown. The influence of specific effects, such as viscous dissipation, axial conduction and variable fluid properties is also discussed. [DOI: 10.1115/1.4005063]

Plate heat exchangers: Recent advances

Abstract: This study presents the advances in plate heat exchangers both in theory and application. It dresses the direction of various technical research and developments in the field of energy handling and conservation. The selected areas of heat transfer performance and pressure drop characteristics, general models and calculations change of phase; boiling and condensation, fouling and corrosion, and welded type plate heat exchangers and finally other related areas are highlighted.

Computational study of heat transfer in a bubbling fluidized bed with a horizontal tube

Abstract: A combined approach of discrete particle simulation and computational fluid dynamics is used to study the heat transfer in a fluidized bed with a horizontal tube. The approach is first validated through the good agreement between the predicted distribution and magnitude of local heat transfer coefficient with those measured. Then, the effects of inlet fluid superficial velocity, tube temperature and main particle properties such as particle thermal conductivity and Young's modulus are investigated and explained mechanistically. The relative importance of various heat transfer mechanisms is analyzed. The convection is found to be an important heat transfer mode for all the studied conditions. A large convective heat flux corresponds to a large local porosity around the tube, and a large conductive heat flux corresponds to a large number of particle contacts with the tube. The heat transfer is enhanced by the increase of particle thermal conductivity while it is little affected by Young's modulus. Radiative heat transfer becomes increasingly important as the tube temperature is increased. The results are useful for temperature control and structural design of fluidized beds. © 2011 American Institute of Chemical Engineers AIChE J, 2012

A Review on Critical Heat Flux Enhancement With Nanofluids and Surface Modification

Abstract: Recently, there has been increasing interest in boiling nanofluids and their applications. Among the many articles that have been published, the critical heat flux (CHF) of nanofluids has drawn special attention because of its dramatic enhancement. This article includes recent studies on CHF increasing during the past decade by various researchers for both pool boiling and convective flow boiling applications using nanofluids as the working fluid. It presents a review of nanofluid critical heat flux research with the aim of identifying the reasons for its enhancement and the limitations of nanofluid applications based on various published reports. In addition, further research required to make use of the CHF enhancement caused by nanofluids for practical applications is discussed. Finally, the surface modification method with micro/nanostructures to increase the CHF is introduced and recommended as a useful way.

Nucleate boiling performance on nano/microstructures with different wetting surfaces.

Abstract: A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.

Heat Transfer Properties of Nanodiamond–Engine Oil Nanofluid in Laminar Flow

Abstract: This study investigates the effects of utilizing nanodiamond as an additive to engine oil on the heat transfer enhancement in laminar pipe flow. A plain tube with internal diameter of 6 mm was used as the test section and heated by an electrical coil heater to produce a constant heat flux thermal boundary condition. Thermal conductivity, specific heat, and viscosity of nanofluids were measured for various volume fractions and temperatures. In addition, convection heat transfer coefficients and Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Peclet and Reynolds numbers. Experimental results clearly indicate the heat transfer enhancement due to the presence of nanoparticles in the fluid. However, pressure-drop measurements showed significant increase due to addition of nanopowder to engine oil.

Boiling heat transfer

Abstract: For the design of heat exchangers in which steam or vapor is generated, knowledge of the laws of boiling heat transfer is required. Steam generators are used in steam power plants, heat pumps, refrigerators, boilers, distilling and rectifying columns. Boiling can occur in static and flowing fluids.

Experimental investigation on heat transfer of a specific fuel (RP-3) flows through downward tubes at supercritical pressure

Abstract: The heat transfer characteristics of a specific hydrocarbon fuel RP-3 flowing through vertically downward miniature tubes (din = 1.8 mm) were experimentally investigated at supercritical pressure (P = 5 MPa, PR = 2.15) based on measured thermophysical properties under the fuel temperature ranged from 373 to 800 K. Test results indicated that: in the initial heating region, wall temperature increased dramatically from the initial heating point and then decreased rapidly at higher heat flux, but this phenomenon diminished when the inlet Reynolds number reached to 10,000. In addition, heat transfer deterioration occurred when thermal acceleration parameter (Kv) was less than 1.5 × 10−8 or buoyancy factor (Bo*) was less than 1.6 × 10−10. Finally, a new heat transfer correlation was developed based on the experimental data and which predicted the heat transfer for RP-3 well.

Heat transfer characteristics in latent heat storage system using paraffin wax

Abstract: An energy storage system has been designed to study the heat transfer characteristics of paraffin wax during melting and solidification processes in a vertical annulus energy storage system. In the experimental study, three important issues are focused. The first one is temperature distribution in the phase change material (PCM) during the phase change processes. The second one is the thermal characteristics of the paraffin wax, which includes total melting and total solidification times, the nature of heat transfer phenomena in melted and solidified PCM and the effect of Reynolds number as inlet heat transfer fluid (HTF) conditions on the heat transfer parameters. The final one is to calculate heat transfer coefficient and effectiveness during solidification process. The experimental results proved that the PCM melts and solidifies congruently, and the melting front moved from the top to the bottom of the PCM container whereas the solidification front moved from bottom to the top along the axial distances in the PCM container. Experiment has been performed for different water flow rates at constant inlet temperature of heat transfer fluid for recovery and use of heat. Time-based variations of the temperature distributions were explained from the results of observations of melting and solidification curves. Charging and discharging processes were carried out. Heat transfer characteristics were studied.