Heat transfer—a review of 1993 literature

Inserting roughness geometry on the heat transfer surface of the collector plate is one of the sound ways to improve the heat transport rate through a solar air heater duct (SAHD). The present study elucidates a collective review of the type of roughness geometry used, the range of parameters investigated, and optimum parameters for the maximum thermal performance. This review article compares thermo-hydraulic performance and enhancement in heat transfer of various experimental and numerical works. The provision of artificial roughness on the collector surface has been effective due to breaking the boundary layer, strong-turbulence generation, and secondary flow formation. Roughness elements like multi v-fashion ribs with gaps, multi arc-fashion ribs with gaps, discrete multiple arc-fashion ribs, and conical ring turbulator with jet-impingement are found to have maximum heat transfer ratio compared to the base model in the respective parametric range. A mathematical model with 90% porous serpentine-wavy wire mesh on a collector plate was found to have thermal efficiency and an effective efficiency of 80% and 74%, respectively. The three-sided ribbed SAHD is far superior (40–48% increase in efficiency) to that of single-sided SAHD. Maximum effective efficiency of 80.12% was observed for a parallel-pass double duct with inclined ribs.

Evaporating water droplets on a heated substrate are investigated in this work. Specifically, the influences of electric fields are studied in the context of the heat flux distribution beneath the droplets as well as the droplet mechanics and resulting shapes and forces. To facilitate a deeper understanding of the problem, both hydrophilic and superhydrophobic droplets are considered for an entire evaporation period with and without electric field effects. Both wetting scenarios show that the net radial directed electric force is directed inward, resulting in a compressive force which influences the droplet shape in such a way that it appears elongated. Conversely, the net vertically directed electric force is determined to be downwardly directed for hydrophilic droplets, pressing the droplet to the surface, whereas it is upwardly directed for the superhydrophobic droplets, representing a lifting force. With regard to the heat transfer to the droplets, only a pronounced electric field effect was observed for the superhydrophobic droplet. For all droplets, the contact line density, representing the ratio of the contact line perimeter to the total base area of the droplet, is determined to be a parameter that unifies the average heat flux from the heater to the droplets. This suggests that the heat transfer to the base of the droplet in the presence of an electric field is dominated by the electric fields influence, or lack thereof, on the contact line density.

A box-type solar cooker was tested and modelled during a long time period in Madrid (Spain). The experimental data were employed to obtain the convective coefficients of the heat transfer model proposed. The model was validated with experimental data, obtaining results with a relative error under 4% during a whole month of temperature measurements. The model was also employed to simulate the performance of the solar cooker for a year in several countries around the world, estimating the number of days per year in which the solar cooker can be operated in each location. The results obtained from the model informed of the high potential of solar cooking in developing countries.

Moving beds acting as bed reactors are quite common in industry. Understanding of the multiphase flow and thermal behaviour in moving beds is important for process design and optimisation. In this work, CFD–DEM approach is adopted to investigate the heat transfer behaviour in a moving bed which is relevant to the raceway region in an ironmaking blast furnace. The solid flow behaviour including flow pattern, velocity, and microscopic properties are investigated. A good agreement of gas temperature distribution between simulation and experiments is achieved. Then the heat transfer behaviour is analysed quantitatively through heat flux and contribution of each heat transfer mode. It reveals that under the conditions considered in this work, particle-fluid convection is dominant, and the contribution of radiation is very small but increases when the bed temperature is high. Solid flow rate and gas flow rate have significant but different influences on the momentum and thermal behaviour in moving beds. The study presented in this work provides a solid base for the further investigation of thermal behaviour in moving beds with the consideration of chemical reactions, and more complicated operational conditions.

The enhancement of heat and mass transfer using a static electric field is an interesting process for industrial applications due to its low energy consumption and potentially high level of evaporation rate enhancement. However, to date, this phenomenon is still not understood in the context of the evaporation of sessile drops. This chapter encompasses the existing literature about the effect of an electrical field established by two plate electrodes on sessile drops. First, the investigations about the influence of the electric field on the contact angle and the drop shape are discussed. Next, the studies about the enhancement of evaporation under an electrical field are summarized. Finally, we conclude with a discussion of possible future investigations.

The enhancement of heat and mass transfer using a static electric field is an interesting process for industrial applications, due to its low energy consumption and potentially high level of evaporation rate enhancement. However, to date, this phenomenon is still not understood in the context of the evaporation of sessile drops. We synthesise the current research concerning the effect of an electric field on sessile drops with a focus on the change of contact angle and shape and the influence of the evaporation rate. We also discuss the future prospects of this research field.