Heat transfer and parameter optimization for medical waste pyrolysis in an indirectly heated rotary kiln.

Experimental study on heat and mass transfer of falling liquid films in converging-diverging tubes with water

Effect of vacuum environment on air side heat and mass transfer characters of plain fin-tube heat exchangers

For electronic systems cooling analysis, it is incomprehensive to evaluate heat transfer performance only by one parameter, such as heat transfer coefficient, heat transfer area, heat transfer temperature difference and so on, especially for the multidimensional heat transfer problems. The parameter, temperature difference weighted with heat transfer capacity (TDW), is proposed to evaluate the heat transfer performance and the mean junction temperature of cooling systems with multiple heat source under the fixed heat flux. The weighted residual approach is employed to analyze the heat potential capacity transfer process in control volume of convection, then the variational principle is used as well to deduce the TDW, which could be used to optimize heat transfer performance in electronic systems cooling and reveal the relationship between temperature difference uniformity and heat transfer performance. Moreover, heat transfer performance could be improved by decreasing the TDW or improving the uniformity of temperature difference in cooling multiple heat sources. According to the theoretical analysis, the lower the TDW, which means the lower the thermal resistance, that is, the convection heat transfer is enhanced and heat transfer efficiency is improved. In order to validate the proposed theory, a new heat sink with slotted and folded fins is presented for the sake of obtaining lower value of the TDW. CFD simulations was conducted to validate the theory with the new heat sink. Simulation results indicate that both the mean junction temperature of heat sources and the TDW of the new designed heat sink with slotted and folded fins are lower than those of conventional heat sinks, so a lower TDW can be used to represent to the higher uniformity of temperature differences. Combined with theoretical calculation and CFD simulation, it could be concluded that for the fixed heat flux, if the TDW is low, uniformity of temperature differences will be high, and the mean junction temperature of heat source will be low as well, the efficiency of heat transfer will be high in convection. The new heat sink also could be used for decreasing the mean junction temperature of multiple heat sources and improving the heat transfer performance of conventional heat sinks under the fixed flux. In electronic systems cooling technology, a new parameter named TDW can be used to analysis the heat transfer performance and heat transfer efficiency.

Microchannel heat sinks have been proven to be an effective means of solving small-scale, high heat flux heat dissipation. This research comprehensively evaluates fluids' heat transfer and flow performance in wavy microchannels with different cross-sectional features by establishing heat and mass transfer models in finite element analysis software. And it is innovatively clarified the relationship between the included angle between the microchannel walls and the fluid heat transfer performance. Different characteristic cross-sections with the same hydraulic diameter included circular, hexagonal, rectangular, trapezoidal and triangular. The microcapsule phase change slurry(MPCS) was the heat transfer fluid with 10 wt%. And the performance evaluation indices included Nusselt number, Stanton number, Euler number, thermal resistance, dimensionless temperature, pressure drop, pump power, and maximum temperature. The research results show that the circular microchannel had the largest heat transfer coefficient and heat transfer efficiency. And the heat transfer coefficient of MPCS in circular microchannels was sensitive to changes in velocity at low Reynolds number. The triangular microchannel had the smallest relative momentum loss rate for flow characteristics. But after considering the cross-sectional area of the microchannel, the circular, hexagonal and trapezoidal microchannels all showed lower power consumption. Furthermore, when the cross-section had acute angles, the maximum temperature of the wall surface increased by 7.5-19.5℃. This is because there is an obvious weak flow velocity area near the acute angle, which significantly reduces the heat transfer coefficient near the wall surface. However, the existence of obtuse angles has little effect on the heat transfer performance of microchannels. So the results can provide theoretical support for the design of microchannel heat sinks.

One of the main obstacles in the design process of absorption chillers is the accurate prediction of the heat and mass transfer coefficients for all the heat exchangers. This refers especially for the absorber, where the lowest heat transfer coefficients are expected due to the coupled heat and mass transfer process during absorption. Appropriate correlations for the calculation of the heat and mass transfer coefficients of horizontal tube bundle absorbers are discussed by literature research in this work and are applied on the heat exchanger’s geometry of a self-designed and constructed absorption chiller. The correlations are discussed concerning their dependence on the solution mass flow rate and the considered physical properties. The results of the correlations are validated with experimental data of the self-designed absorption chiller. It is shown that the dependence of the heat transfer coefficient on the material properties is taken into account in the same way in all correlations. In contrast to this, there is no agreement in the correlations on which way the mass transfer is affected by the material properties. Due to different flow regimes in the film which are considered in particular correlations, no uniform influence of the solution mass flow rate on the heat and mass transfer is identified. Basically all correlation over estimate heat transfer coefficients or do not show the same dependence of the heat transfer coefficient on the solution mass flow rate compared to experimental data. Only the results of one of the correlations for heat transfer are suitable, but only for low solution flow rates.

Pyrolysis technology currently serves as a significant method for recycling and reducing waste tires. In this paper, in order to improve the heat transfer efficiency during the pyrolysis of waste tires in a horizontal rotary furnace and the yield of pyrolysis oil, the effect laws of tire particle size, rotary furnace rotation speed, enhanced heat transfer materials, and adding spiral fins on heat transfer performance and pyrolysis product distribution were studied, respectively. The innovation lies in two aspects: first, aiming at the problems of slow heat transfer and low pyrolysis efficiency in horizontal rotary furnaces, we identified technical measures through experiments to enhance heat transfer, thereby accelerating pyrolysis and reducing energy consumption; second, with the goal of increasing high-value pyrolysis oil yield, we determined optimal operating parameters to improve economic and sustainability outcomes. The results showed that powdered particles of waste tires were heated more evenly during the pyrolysis process, which increased the overall heat transfer coefficient and the proportion of liquid products. When the rotational speed of the rotary pyrolysis furnace exceeded 2 rpm, there was sufficient contact between the material and the furnace wall, which was beneficial to the improvement of heat transfer performance. Adding heat transfer enhancement materials such as carborundum and white alundum could improve the heat transfer performance between the pyrolysis furnace and the material. Notably, a rotational speed of 3 rpm and carborundum were used as a heat transfer enhancement material with powdered waste tire particles during the pyrolysis process; the overall heat transfer coefficient was the highest, which was 16.89 W/(m2·K), and the proportion of pyrolysis oil products was 46.1%. When spiral fins were installed, the comprehensive heat transfer coefficient was increased from 12.78 W/(m2·K) to 16.32 W/(m2·K). The experimental results show that by increasing the speed of the pyrolysis furnace, adding heat transfer enhancing materials with high thermal conductivity to waste tires, and appropriate particle size, the heat transfer performance and pyrolysis rate can be improved, and energy consumption can be reduced.

Experimental study on the heat and mass transfer characteristics of air-water two-phase flow in an evaporative condenser with a horizontal elliptical tube bundle

Experiments were conducted to investigate the heat transfer and flow characteristics of the vertical upward smooth and rifled tubes from subcritical to supercritical pressure. The distributions of wall temperature and heat transfer coefficient (HTC) were obtained, and the HTC correlations and friction resistance coefficient correlations were fitted with experimental data. In addition, the influences of heat flux and type of tube on heat transfer performance were analyzed. The research shows that heat flux has different influences on the heat transfer characteristics under different pressures. The increase in heat flux improves the heat transfer characteristics in the nucleate boiling region, yet it leads to the advance in heat transfer deterioration. However, for supercritical water, the increase in heat flux reduces the heat transfer ability. In addition, using the rifled tube not only improves the heat transfer performance, but also inhibits the occurrence of heat transfer deterioration. The fitted correlations have great predictive ability for the heat transfer coefficient and friction resistance coefficient, and the average relative fitting errors are limited to 20%.

In order to increase entry gas temperature and improve the efficiency of gas turbine, steam is used as a coolant instead of air. Much research has been carried out on the closed circuit steam cooling of vanes substituted with film-cooling using compressor air in recent years. Furthermore, by studying the steam flow and heat transfer characteristics in rib ducts, this investigation focuses on establishing the basis of steam cooling technology application in complex flow field of internally-cooled turbine vane. In this paper, a report and assessment of RSM method based on SSG turbulence model is performed with commercial computational fluid dynamics software ANSYS CFX. The numerical results of heat transfer coefficient and friction factors in square channels with 90 degree rib turbulators for Reynolds numbers of 10 000, 30 000 and 60 000 are compared with the experimental data from Han’s. It is found that the obtained heat transfer coefficient distributions and friction factors match well with SSG turbulence model. In addition, the heat transfer distribution and pressure drop of steam-cooled ducts are predicted under the same work conditions by using dry real gas model. The Reynolds number could be correlated with the Nusselt number. The impact of steam physical properties on heat transfer performance are researched detailedly by respectively changing the steam superheat and entry pressure. The results indicate that the RSM method with a suitable turbulence model is valuable for the air-cooled and steam-cooled duct with the acceptable engineering accuracy (less than 20%). Comparing the cooling efficiency between steam and air under the same operation condition, the advantage of using cooling steam is evident than using cooling air. Furthermore, the efficiency of the whole gas turbine system will be greatly improved through using the closed loop steam cooling system. Changing the steam superheat and entry pressure, it has little effect on the steam flow and heat transfer characteristics. Increasing the steam overheat would raise the friction factor. Contrarily, enhancing the entry pressure would decrease the friction factor.

The development of hydrophilic surface coatings for enhanced wetting characteristics has led to improvement in heat transfer metrics like impinging droplet vaporization time and the heat transfer coefficient. Hydrothermal synthesis, a method of developing hydrophilic surfaces, has been previously shown to produce high performing heat transfer surfaces on copper substrates [1]. Our study applied this production method to aluminum substrates, which have the advantage of being cheaper, lighter, and a more widely used for heat sinks than copper. Previous experiments have shown that water droplets on ZnO nanostructure coated surfaces, at low superheats, evaporate via thin film evaporation rather than nucleate boiling. This leads to heat transfer coefficients as much as three times higher than nucleate boiling models for the same superheat. Our nanocoated aluminum surfaces exhibit superhydrophilicity with an average droplet liquid film thickness of 20–30 microns, which can produce heat transfer coefficients of over 25 kW/m2K. This study discusses characterization of ZnO nanostructured aluminum surfaces to better understand the related mechanisms which lead to such high heat transfer performance. All ZnO nanostructured aluminum surfaces produced for this study exhibited superhydrophilicity, with sessile droplet contact angles of less than 5 degrees. The challenge of achieving accuracy for such low contact angles led to the development of a new wetting metric related to the droplet’s wetted area on a surface rather than the contact angle. This new metric is predicated on the the fact that heat transfer performance is directly related to this wetted area, thickens, and shape of the expanding droplet footprint. Shape irregularity of droplets on these superhydrophilic surfaces is discussed in this study, where there appears to be advantages to irregular spreading compared with surfaces that produce symmetric radial spreading. One form of irregular spreading consists of liquid droplets spreading out both on top of the surface and within the microstructure of the surface coating. The liquid within the microstructure forms films less than 5 microns thick, making local heat transfer coefficients of greater than 100 kW/m2K possible. SEM microscope imaging provided additional insight to the underlying mechanisms which cause these surfaces to produce such exceptional spreading as well as irregular spreading, resulting in very good heat transfer performance. Experimental work was coupled with computational analysis to model the contact line of the droplet footprint. Image processing of experimental photos helps to analyze spreading characteristics, which can be directly related to heat transfer due to film thickness at various points during spreading. Approaches used to characterize these superhydrophilic surfaces advance understanding of the connections between nanoscale structural elements and macroscale performance characteristics in heat transfer. This understanding can reveal key insights for developing even better high performance surfaces for a broad range of applications.

Pool boiling in porous media has been applied in various thermal management systems by using latent heat and increasing the heat transfer area and thermal conduction path to improve the heat transfer performance. In mechanical equipment, vibration is an inevitable problem due to reasons such as engine operation and high-speed relative motion between transmission system components, which causes the system components to be affected by vibration forces or vibration accelerations. This study focuses on a review of published articles about the effects of mechanical vibration on the characteristics of boiling process in porous media by two aspects: heat transfer performance and bubble dynamics. Heat transfer coefficient (HTC) and critical heat flux are two main parameters used to measure the boiling heat transfer characteristics of porous media. For bubble dynamics investigations, properties such as migration, fragment, coalescence, departure diameter and frequency are the focus of research attention. Different mechanical vibration parameters, i.e., direction, frequency, and amplitude, will have different effects on the above characteristics. It is worth mentioning that the greatest influence occurs under resonance conditions, and this has been verified through experimental and simulation calculations. This review highlights the importance of considering mechanical vibrations in the design and optimization of porous media systems for efficient heat transfer applications. Further research is warranted to explore the detailed mechanisms and optimize the vibration parameters for enhanced heat transfer performance in thermal management systems using porous media.

Heat transfer enhancement is a significant requirement in process industries. There are various passive and active methods available to reinforce the heat transfer. In the present study, tangential injection and its effect on the advancement in heat transfer rate are analysed. This is an active technique for enhancing heat transfer accompanied by negligible increment in pressure drop. Design and operating parameters like geometry, pipe material, working fluid, and flow rate play a vital role in heat transfer characteristics. Taking into account the design and operating parameters, the present study is aimed to optimize the orientation of the injecting nozzle. FLUENT software is employed to simulate the heat transfer and flow characteristics. A thorough grid independence study has been administered, and the present results are validated using the author’s previous experimental work. The tangential orientation considered is 30°, 45°, and 60°. The results show 60° inclination of the nozzle for parallel flow gives maximum overall heat transfer coefficient.

Improvement in heat transfer and flow pattern of sprayed falling on horizontal tubes with rib structure for a sewage source heat pump

In this research article, experimental study was carried out to obtain the heat transfer characteristics between a submerged horizontal tube bundle and a fluidized bed in a large-scale circulating fluidized bed (CFB) boiler with an external heat exchanger (EHE). The operational parameters in the tube EHE were measured during performance tests at variable load conditions. The average heat transfer coefficient (HTC) was calculated using a mechanistic heat transfer model based on packed renewal theory. The heat transfer characteristics are considered in terms of heat transfer mechanisms such as emulsion phase convection, gas convection and also thermal radiation. The obtained heat transfer data exhibit a maximum value with variation mean bed particle size irrespective of pressure. The results showed that the average HTC increases with a decrease of the Sauter mean particle diameter and with the increase of the fluidizing number as a result of good mixing dynamics in emulsion phase (i.e. emulsion wall contact time, bubble fraction in the bed). Based on the heat transfer data, empirical correlations are proposed for predicting a heat transfer coefficient from fluidized bed to horizontal tube bundle. The mechanistic heat transfer model predicted the average HTC in sufficiently good agreement with CFB boiler data accessible in the literature.

Effect of rib orientation on heat transfer and flow characteristics of mist/steam in square channels