Local Heat Transfer Mechanisms Research Articles

Modelling of heat and mass transfer in clothing considering evaporation, condensation, and wet conduction with case study

Over the past few decades, several models of heat and mass transfer have been based on the assumption of the homogeneous thickness (or absence) of the enclosed air layer contradicting the real-life scenario of the skin-clothing-environment system. To address this research gap, in this study, the comprehensive model for heat and mass transfer was developed which considers thermal phenomena such as evaporation, condensation, and the effect of liquid moisture on the thermal insulation of fabrics (wet conduction) along with sensible heat transfer mechanisms and effects of spatial heterogeneity of air layers. The coupling between heat and mass transfer in the modelling approach resulted in an accurate evaluation of the mass of evaporated liquid moisture and condensed water vapour in a skin-clothing-environment system. The developed model is systematically validated with an increasing level of spatial complexity using homogeneous and heterogeneous air layers with single-layer and multi-layer clothing ensembles. The model is validated for 21 different experimental cases covering various parameters that affect mass transfer such as enclosed air gap thickness (clothing fit), evaporative resistance (permeable, semi-permeable, and impermeable to water vapour), ambient temperature (−10 °C to 34 °C), and relative humidity (80% to 18.5%). Finally, the relevance of the model for product development is demonstrated by a case study about smart skiwear. The presented study provides detailed insights into individual heat and mass transfer mechanisms in the skin-clothing-environment system, which is very useful to develop an in-depth understanding about local heat transfer mechanisms and for designing and development of functional and protective clothing.

3D heat transfer analysis in a loop heat pipe evaporator with a fully saturated wick

A practical quasi three-dimensional numerical model is developed to investigate the heat and mass transfer in a square flat evaporator of a loop heat pipe with a fully saturated wicking structure. The conjugate heat transfer problem is coupled with a detailed mass transfer in the wick structure, and incorporated with the phase change occurring at the liquid–vapor interface. The three-dimensional governing equations for the heat and mass transfer (continuity, Darcy and energy) are developed, with specific attention given to the wick region. By comparing the results of the numerical simulations and the experimental tests, the local heat transfer mechanisms are revealed, through the obtained temperature distribution and the further derived evaporation rates along the liquid–vapor interface. The results indicate that the model developed herein can provide an insight in understanding the thermal characteristics of loop heat pipes during steady-state operation, especially at low heat loads.

Heat Transfer in a Supercritical Fluid: Classification of Heat Transfer Regimes

Because of the dramatic variation of physical properties with a modest change of temperature, no existing engineering correlation or models can accurately predict heat transfer of supercritical fluids. This paper seeks to classify the conditions where the existing models are applicable and to better understand these local heat transfer mechanisms. The first objective is the focus of this paper. FLUENT was employed to compute the wall temperatures for various heat flux and mass flux conditions and to be compared with experimental data. Because the model was developed for a wide range of flow conditions, it was necessary to make certain assumptions. The simulations showed a good agreement with high mass flux conditions, where buoyancy effects could be neglected. The FLUENT model, however, had difficulty predicting the localized low heat transfer rates seen in the combined condition of high heat flux and low mass flux. A new generalized parameter, dependent on the heat and mass flux, was developed to classify under which conditions this FLUENT standard model was applicable. This global Froude number can be used as the parameter to predict under which conditions the buoyancy effect will be dominant and lower heat transfer rates will occur.

A study of local heat transfer mechanisms along the entire boiling curve by means of microsensors

An array of 36 microthermocouples (38 μm diameter) embedded in a horizontal copper heater (distance to the surface 3.6 μm), a micro optical probe (tip diameter ∼1.5 μm) and a microthermocouple probe (tip diameter ∼16 μm), both moveable above the heater surface, are applied to study heat transfer mechanisms along the entire boiling curve under steady-state conditions. Test fluids are isopropanol and FC-3284. In nucleate boiling, very localized and rapid temperature drops are observed indicating high heat fluxes at the bottom of the bubbles. Already before reaching CHF, hot spots occur the size of which increases towards the Leidenfrost point. In the entire transition boiling regime wetting events are observed, but no ones in film boiling. In low heat flux nucleate boiling small vapor superheats exist in the bubbles and strong superheats in the surrounding liquid. This characteristic changes continuously with increasing wall superheat: the liquid surrounding the vapor approaches saturation whereas the vapor becomes more and more superheated. In film boiling the bubbles leaving the vapor film can reach superheats of 40 K near the surface. The optical probes confirm a liquid rich layer near the surface between nucleate boiling and high heat flux transition boiling. The void fraction in this layer increases continuously with the distance to the surface until a maximum value which seems to be linked to the bubble departure diameter.

Instantaneous local heat transfer and hydrodynamics in a circulating fluidized bed

Local heat transfer mechanisms and hydrodynamics are studied in a 9.3 m tall, 152 mm i.d. transparent cold model circulating fluidized bed for 171 μm Ottawa sand. Instantaneous measurements at the wall are made with platinum-coated heat transfer probes. For some conditions, simultaneous local voidages are determined using a capacitance probe. Results show that the sudden and dramatic peaks in the measured instantaneous heat transfer coefficients are directly caused by the arrival of strands of particles at the heat transfer surface. Analyses of the capacitance probe signals indicate that these strands possess wide distributions of voidages which vary with the local time-averaged area-averaged suspension density. Simultaneous heat transfer probe measurements further suggest the existence of characteristic residence lengths for these strands. The average falling velocity of the strands is 1.26 m s−1 using high-speed cinematography.