Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow

In this study, a three-dimensional computational method was developed to predict convective heat transfer combined with water phase change/two-phase flow in a PEMFC cathode duct. Momentum, heat transport and species equations have been solved by coupled source terms and thermo-physical properties of the multi-component mixture. Advanced boundary conditions are applied at the cathode duct external walls in the analysis, i.e., combined thermal boundary conditions of heat flux on the active surface and thermal insulation on the remaining solid walls. Moreover, effects of mass consumption and generation appearing on the active surface are implemented. The calculated results reveal that the thermal conditions at the interfaces vary along the main stream, and a big permeability of the porous layer promotes the bulk gas transport and the generated liquid water removal via the bottom interface between the flow duct and the porous layer, and a big convective heat transfer coefficient as well.

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ANALOGY BETWEEN WALL SHEAR STRESS AND CONVECTIVE HEAT TRANSFER COEFFICIENT IN TWO PHASE FLOW

Proceeding of International Heat Transfer Conference 7 ◽

10.1615/ihtc7.4540 ◽

1982 ◽

Author(s):

J. M. Chawla

Keyword(s):

Heat Transfer ◽

Shear Stress ◽

Heat Transfer Coefficient ◽

Wall Shear Stress ◽

Transfer Coefficient ◽

Convective Heat Transfer ◽

Convective Heat ◽

Convective Heat Transfer Coefficient ◽

Phase Flow ◽

Two Phase

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Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini- and Microchannel Heat Exchangers for High Heat Flux Electronics Cooling

10.5772/intechopen.96015 ◽

2021 ◽

Author(s):

Jaime Rios ◽

Mehdi Kabirnajafi ◽

Takele Gameda ◽

Raid Mohammed ◽

Jiajun Xu

Keyword(s):

Heat Transfer ◽

Convective Heat Transfer ◽

Heat Transfer Enhancement ◽

Convective Heat ◽

Phase Flow ◽

Transfer Performance ◽

Transfer Enhancement ◽

Heat Transfer Characteristics ◽

Two Phase ◽

Flow And Heat Transfer

The present study experimentally and numerically investigates the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid, namely, ethanol/polyalphaolefin nanoemulsion, inside a conventionally manufactured minichannel of circular cross section and a microchannel heat exchanger of rectangular cross section manufactured additively using the Direct Metal Laser Sintering (DMLS) process. The experiments were conducted for single-phase flow of pure polyalphaolefin (PAO) and ethanol/PAO nanoemulsion fluids with two ethanol concentrations of 4 wt% and 8 wt% as well as for two-phase flow boiling of nanoemulsion fluids to study the effect of ethanol nanodroplets on the convective flow and heat transfer characteristics. Furthermore, the effects of flow regime of the working fluids on the heat transfer performance for both the minichannel and microchannel heat exchangers were examined within the laminar and transitional flow regimes. It was found that the ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of the pure PAO base fluid under both single- and two-phase flow regimes. While the concentration of nanoemulsion fluids did not reflect a remarkable distinction in single-phase heat transfer performance within the laminar regime, a significant heat transfer enhancement was observed using the nanoemulsion fluids upon entering the transitional flow regime. The heat transfer enhancement at higher concentrations of nanoemulsion within the transitional regime is mainly attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid. For two-phase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced when the ethanol nanodroplets underwent phase change. A comparative study on the flow and heat transfer characteristics was also implemented between the traditionally fabricated minichannel and additively manufactured microchannel of similar dimensions using the same working fluid of pure PAO and the same operating conditions. The results revealed that although the DMLS fabricated microchannel posed a higher pressure loss, a substantial heat transfer enhancement was achieved as compared to the minichannel heat exchanger tested under the same conditions. The non-post processed surface of the DMLS manufactured microchannel is likely to be the main contributor to the augmented heat transfer performance. Further studies are required to fully appreciate the possible mechanisms behind this phenomenon as well as the convective heat transfer properties of nanoemulsion fluids.

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