Effects of titania nanoparticles on heat transfer performance of spray cooling with full cone nozzle

Spray cooling using aqueous titania nanofluids was studied. The temperatures of a testing plate under various spraying conditions were first measured; an inverse heat conduction technique was then applied to convert these measured temperatures into heat transfer coefficients (HTCs). It was found that the HTC increased logarithmically with the volume flux, but was decreased with the increase of the nanoparticle fraction. A correlation analysis was performed to quantify the HTC reduction caused by the increase of nanoparticles, and reconfirmed that the major cause for the HTC reduction was the difference in the impact (or impingement) behavior between solid nanoparticles and fluid droplets. A comparison study of the present findings with the previous published results was also performed and indicated that all results compared were consistent to each other based on the similar spray cooling conditions with different nanofluids or nozzles. The effects by using aquatic titania nanofluids instead of aquatic alumina nanofluids and by using full-cone nozzle instead of solid jet nozzle were specifically assessed and the associated rationales for the differences in these effects were given.     

无沸腾喷雾冷却中流量和喷头高度对换热性能的影响

采用薄膜电阻加热器进行了喷头进口压力，喷头类型，喷头高度对换热系数影响的实验研究。研究了冷却介质的质量流量对换热性能的影响，并测量了同一喷头在不同喷头高度下的换热系数大小。实验发现当喷雾面积近似等于实验用薄膜加热器面积时冷却能力达到最大。根据以上实验结果可以最优化喷雾冷却性能。     

Optimal design of a micro evaporator with lateral gaps

This paper presents an optimal design of a micro evaporator, to maximize the heat transfer coefficient (HTC) and it forms the starting point in developing miniaturized vapor–compression refrigeration system. The experimental design is adopted to determine the optimal parameters of the evaporator for realizing the inlet–outlet conditions of the refrigerating cycle, and for increasing the HTC. The number of lateral gaps, channel width, and lateral gap size were optimized to maximize HTCs of 2062, 2029, and 1895 W/m²K for heating powers of 40, 60, and 80 W, respectively. The refrigerant and the mass flow rate were fixed as R-123 and 0.72 g/s, respectively. Among the three design parameters, the channel width is the most sensitive parameter influencing the HTC. A periodic change of flow pattern was observed in the evaporator with high HTCs, and a dryout was observed in the evaporator with low HTCs.     

The Inverse Reconstruction of the Heat Transfer Coefficient for the Free Surface Water Jet

This article presents an application of inverse algorithm for reconstruction of heat transfer coefficient (HTC) for a water jet impinging a flat surface. Such an approach, allows for decoupling complex fluid flow from heat conduction in a solid impinged by jet. The approach starts with parameterization of a functional form of unknown boundary temperature and heat flux occurring at the fluid–solid interface. Later, Newton's law of cooling is used to force temporal invariability of HTC. Unknown coefficients of HTC distribution are determined from a least square fit between measured and computed temperatures. Temperatures entering the objective function are recorded by an infrared camera at the surface opposite to impinged one.     

Thermodynamic investigation on gasification performance of sewage sludge-derived hydrochar: Effect of hydrothermal carbonization

Compared with the conventional thermal drying process, hydrothermal carbonization (HTC) can reduce the energy cost of water removal from sewage sludge prior to its steam gasification. However, less attention is paid on the interactions between HTC and gasification. In this study, the thermodynamic evaluation on hydrochar gasification performance under different operating conditions including HTC duration (τ), HTC temperature (T_HTC), gasification temperature (T_g), and steam/hydrochar mass ratio (S/C ratio) is performed. Two indicators including carbon conversion rate (CC) and cold gas efficiency (CGE) are used to assess the gasification performance. The results show that elevating both gasification temperature and S/C ratio can enhance the H₂ production, which also result in the increase of CC and CGE. The content and gasification activity of fixed carbon increase under moderate HTC duration and temperature, favoring the H₂ formation despite of the apparent loss of volatiles species in the hydrochar. Longer HTC duration or higher HTC temperature declines the H₂ production due to the sharp reduction of carboxyl and hydroxyl groups, weakening water gas reaction and on-site reforming reaction of tar occurred on the hydrochar surface. In terms of the values of CC = 93.9% and CGE = 64.38%, the optimum HTC conditions of τ = 30min and T_HTC = 200 °C can be determined. The data provided here favor guiding HTC treatment of sewage sludge targeting gasification and thus promoting the development of this promising waste-to-energy technology.     

Characterization of water gas shift reaction in association with carbon dioxide sequestration

The characteristics of a water gas shift reaction (WGSR) in association with carbon dioxide sequestration under the effects of a high-temperature catalyst (HTC) and a low-temperature catalyst (LTC) are studied experimentally. With the condition of fixed residence time (0.1 s) for the reactants in the catalyst bed, it is found that the reaction behaviors with the HTC are inherently different from those with the LTC. Specifically, for the WGSR with the HTC, the reaction can be divided into a rapid growth regime, a progressive growth regime and a slow growth regime with increasing reaction temperature or steam/CO ratio. With regard to the WGSR with the LTC, three different regimes are also exhibited; however, they consist of a rapid growth regime, a progressive decay regime and a growth-frozen regime. According to the aforementioned characteristics, proper or better operation conditions using the HTC and the LTC for the application of fuel cells are suggested. When the product gas passes through a Ca(OH)₂ solution, the obtained results reveal that CO₂ removal efficiency increases with increasing solution concentration or steam/CO ratio for both the HTC and the LTC used in the WGSR.     

Transient thermoelastic analysis of infinite plates with time-periodic heat transfer coefficient: Frequency response of thermal stress

An analytical method is presented for one-dimensional transient heat conduction and associated thermal stress problems of a layered infinite plate whose heat transfer coe?cient (HTC) on one external boundary is an arbitrary function of time. A shifting function method and an orthogonal expansion technique are used to obtain analytical solutions to the problems. Using the solutions, the structural response to sinusoidal HTC oscillation is investigated in single and bilayered plates, the other boundary of which is kept at constant temperature. Numerical results demonstrate the effects of average HTC (measured by the Biot number) and mechanical boundary conditions on the frequency responses of temperature and thermal stress. It is found that the range of stress oscillation decreases with an increase in the frequency of HTC oscillation except for single-layered plates without surface tractions.     

Robust methodology for determination of heat transfer coefficient distribution in convection

To predict the microstructures, residual stresses and distortions in the heat treated metal components, it is important to accurately know the heat transfer coefficients (HTCs) between the hot work piece and cooling media. In this paper, a new method is presented to accurately determine the node-based HTC distribution by coupling computational fluid dynamics (CFD) with optimal weight functions and scale factors. With this new method, the predicted temperature profile of the work piece during quenching (rapid cooling) is in excellent agreement with experimental measurements. This new method can be also applied to accurately predict convection heat transfer in thermal equipment such as heat exchangers and refrigerators, building thermal design and other heat transfer related situations.     

Thermodynamic assessment of modified Organic Rankine Cycle integrated with parabolic trough collector for hydrogen production

Hydrogen is one of the most clean energy carrier and the best alternative for fossil fuels. In this study, thermodynamic analysis of modified Organic Rankine Cycle (ORC) integrated with Parabolic Trough Collector (PTC) for hydrogen production is investigated. The integrated system investigated in this study consists of a parabolic trough collector, a modified ORC, a single effect absorption cooling system and a PEM electrolyzer. By using parabolic trough collector, solar energy is converted heat energy and then produced heat energy is used in modified ORC to produce electricity. Electricity is then used for hydrogen production. The outputs of this integrated system are electricity, cooling and hydrogen. By performing a parametric study, the effects of design parameters of PTC, modified ORC and PEM electrolyzer on hydrogen production is evaluated. According to the analysis results, solar radiation is one of the most important factor affecting system exergy efficiency and hydrogen production rate. As solar radiation increases from 400?W/m² to 1000?W/m², exergy efficiency of the system increases 58%–64% and hydrogen production rate increases from 0.1016?kg/h to 0.1028?kg/h.     

Numerical investigation of the flow field and heat transfer characteristics for upstream continuous and truncated ribs

To verify the applicability of upstream ribs in film cooling, the present numerical study examines heat transfer characteristics and flow field for ribs located upstream of the film hole. Five ribs including bilaterally truncated ribs, centrally truncated ribs, and continuous ribs are explored with the smooth case at two blowing ratios and fixed crossflow Reynolds number. The results show that the film cooling effectiveness of cases with ribs outperforms the case without rib at a low blowing ratio. Centrally truncated ribs and continuous ribs provide superior cooling effectiveness than bilaterally truncated ribs and smooth cases. The introduction of ribs makes the distribution of the heat transfer coefficient (HTC) uneven after the hole. Among these, centrally truncated ribs increased the HTC, while bilaterally truncated ribs reduce the HTC in the far hole area at a high blowing ratio. It is found that anti-kidney-shaped vortex pairs are generated between two adjacent jets for centrally truncated rib cases, while they are generated in front of the jets for bilaterally truncated rib cases. For continuous rib, the impingement of the mainstream gas on the jet leads to a reduction in strength of the kidney-shaped vortex, which allows the coolant to form a better coverage.     

Thermal investigation of the conjugate heat transfer problem in multi-row circular minichannels

A numerical three-dimensional flow and conjugate heat transfer in circular minichannel-based multi-row heat sink is presented in this article. Effects of geometrical parameters including channel dimensions, channel arrangements (inline or staggered), and the number of channel rows with a single-pass flow on the thermal performance of the heat sink are presented. The determination of the bottom surface temperature, average heat transfer coefficient, thermal resistance as well as the pressure drop was reported. The number of rows and the diameter of the circular channel for a constant Reynolds number were found to have a remarkable cooling effect on the heat sink. It was found out that in the case of using four channel rows with the channel diameter of 1?mm, the cooling capacity is 88.5?W/cm² compared to 28?W/cm² for a single row 1?mm diameter.     

Performance evaluation of a vertical ground‐source heat pump system in Izmir,Turkey

This study summarizes the cooling performance of a ground‐source heat pump system which was installed in a 65‐m² room in the Solar Energy Institute, Ege University, Izmir (568 degree‐days cooling, base: 22°C; 1226 degree‐days heating, base: 18°C) Turkey. The institute, built in 1986, has a liveable floor area of 3000 m², and uses passive solar techniques. The heating and cooling loads of the room were, respectively, 3.8 and 4.2 kW at design conditions. The system was commissioned in May 2000 and the performance tests have been carried out since then. Based upon the measurements, the heat rejection rate to the soil with an average thermal diffusivity of 0.00375 m²h^?1 in the cooling mode was found to be in average 51 W m^?1 of bore depth, while the maximum entering water temperature to the unit was recorded as 35.9°C. The cooling coefficient of performance of the heat pump and the whole system was relatively low when compared to other heat pumps operating under conditions at or near design values. The primary reasons for this were discussed in detail and the potential for performance improvements was also suggested. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat capacity and thermal conductivity considerations for coal particles during the early stages of rapid heating

Heating and cooling transients for a number of individual coal particles in the 100-μm size range were measured under rapid heating conditions (10⁴–10⁵ K/s heating rate). In addition to temperature measurements, each particle was fully characterized with respect to external surface area, volume, mass, and density prior to heating. Measured temperatures were compared with model predictions and a sensitivity analysis was performed to critically evaluate model assumptions regarding particle thermal properties. Simulations using temperature-dependent heat capacity and thermal conductivity correlations routinely applied to coal severely under predicted the particle temperature rise during the early stages of heating. Simulations using constant room temperature values for heat capacity and thermal conductivity showed excellent agreement with measurements during the early stages of heating. Increases in coal heat capacity and thermal conductivity reported in the literature are observed under slow heating conditions and result from bond breaking and structural changes which lead to an increase in vibrational modes of freedom in the coal structure. Results of the present study suggest that under rapid heating conditions the coal structure is frozen and that these vibrational modes only become accessible at higher temperatures or longer soak times. These considerations are important if one desires to accurately model the combustion behavior of coals.     

Thermal hydraulic analysis of the HECLIC blanket breeder unit for CFETR

China Fusion Engineering Test Reactor (CFETR) is proposed by the China National Integration Design Group. A helium‐cooled lithium ceramic (HECLIC) blanket for CFETR has been designed by Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) and University of Science and Technology of China (USTC). The HECLIC blanket includes the tungsten armor, first wall (FW), breeder units (BUs), caps, stiffening grids and back plates. The BU consists of the beryllium pebble beds, lithium ceramic pebble beds and cooling plates. The stiffening grid reinforces the blanket structure and also cools the BU together with cooling plates. Thermal hydraulic analysis has been performed to assess the heat transfer coefficient (HTC) and temperature distribution. The results indicate that the velocity is fairly uniform in the cooling loops, and the HTC is high enough to remove the heat timely. The maximum temperatures are within the design temperature limits. Besides, optimization has been done to improve the layout of the cooling channels. In addition, thermal mechanical analysis has been carried out. The maximum stress can satisfy design limits, and it proves the feasibility of the design. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of hydrothermal carbonization and chemical activation process conditions on hydrogen storage in loblolly pine-derived superactivated hydrochars

While the challenge of storing hydrogen in inexpensive and renewable adsorbents is relentlessly pursued by researchers all over the world, application of hydrochar derived from biomass is also gaining attention as it can be subsequently chemically activated using activating agents like KOH in order to tailor the development of favorable porosity. However, the synergistic effect of hydrothermal carbonization (HTC) process conditions as well as KOH activating conditions on the development of surface morphology is required to be assessed with the application of such porous superactivated hydrochars in hydrogen storage application. In this study, highly porous superactivated hydrochars were fabricated from inexpensive and abundant loblolly pine. Loblolly pine was hydrothermally carbonized at 180 °C, 220 °C and 260 °C and the hydrochars were then activated at different experimental conditions of 700 °C, 800 °C and 900 °C using solid KOH to loblolly pine hydrochar ratio of 2:1, 3:1 and 4:1 to produce superactivated hydrochars. Superactivated hydrochars as well as loblolly pine and its corresponding hydrochars underwent physicochemical analysis as well as surface morphology analysis by SEM and nitrogen adsorption isotherms at 77 K in order to investigate the effect on BET, pore volume, and pore size distribution due to various process conditions. The superactivated hydrochars were then analyzed to quantify total hydrogen storage capacity of these materials at 77 K and up to pressure of 55 bar. Porosity of superactivated hydrochars were as high as 3666 m²/g of BET specific surface area (SSA), total pore volume of 1.56 cm³/g and micropore volume of 1.32 cm³/g with the hydrogen storage capacity of 10.2 wt% at 77 K and 55 bar. It was conclusive from principal component analysis that higher HTC temperature with moderate activation condition demonstrated favorability in developing porous superactivated hydrochars for hydrogen storage applications.     

Free convection in inclined air layers heated from above

An experimental investigation of steady-state natural convection heat transfer was carried out in finite rectangular air layers heated from above. Two different aspect ratios, namely A = 20 and 80, and perfectly conducting boundary conditions on the end walls were used. The angle of inclination was varied from Φ = 0 (heated from below) to Φ = 180° (heated from above). A total of 226 test points were taken for heat transfer measurements in air layers heated from above at four different orientations in the range 120 ? Φ ? 180° for Rayleigh numbers between 10² and 2 × 10⁶. Additional test points have been carried out to show the effect of the angle of tilt in the range O ? Φ ? 180° on the average Nusselt number for fixed values of the Rayleigh number. Local measurements of the Nusselt number over discrete portions of the air layer are reported to show the Nusselt number distribution over different flow regimes.     

Experimental investigations on shell and helical coil solution heat exchanger in NH3-H2O vapour absorption refrigeration system (VAR)

This paper presents the performance investigation of a shell and helical coil type of Solution Heat Exchanger (SHX) in an ammonia–water vapour absorption system. In an absorption system, SHX is one of the major heat recovery components. The main objective of any heat exchanger design is to achieve minimum heat transfer area required for a given heat duty, as it governs the overall fixed cost content of such a system. The required surface area is decided by the overall heat transfer coefficient. Hence, the heat transfer coefficient (HTC) correlation plays a major role in optimizing the heat exchanger. In this paper, shell and helical coil type of SHX is investigated with more emphasis on the dimensionless correlation of shell side co-efficient, which decides the overall HTC and the size of heat exchanger. From the experimental study, shell side heat transfer coefficient of 510–650 W/m² K is obtained with the heat exchanger effectiveness of 0.84–0.9 for the tested conditions. A proposed Nusselt number correlation is compared with the experimental results.     

Sludge stabilization and energy recovery by hydrothermal carbonization process

Hydrothermal carbonization (HTC) is a thermal conversion process that converts high-moisture biomass into hydrochar. HTC was applied to stabilize and process sludge collected from septic tanks into hydrochar for practical energy recovery. Experiments were conducted with a 1-L high-pressure reactor operating at different temperatures and reaction times in which the sludge was mixed with catalysts and biomass at different ratios. The effects of catalysts (i.e., acetic acid, lithium chloride, borax, and zeolite) and biomass (i.e., cassava pulp, dried leaves, pig manure, and rice husks) mixing with sludge for hydrochar production were investigated. The experimental data showed acetic acid and cassava pulp to be the most effective catalyst and biomass, respectively, increasing energy contents to the maximum value of 28.5 MJ/kg. The optimum HTC conditions were as follows: sludge/acetic acid/cassava pulp mixing ratio of 1/0.4/1 (by weight), at a temperature of 220 °C, and reaction time of 0.5 h. The relatively high energy contents of the produced hydrochar suggest its applicability as a solid fuel.     

Enhancing the onset of pool boiling by wettability modification on nanometrically smooth surfaces

Pool boiling experiments are performed with degassed water on highly smooth surfaces of two different wettabilities: hydrophilic and hydrophobic cases. Boiling curves and visual observations on the boiling have been performed. The onset of nucleate boiling (ONB) has been measured and the influence of the wettability has been quantified. As the inherent mean roughness of the glass substrates was lower than one nanometer it was possible to show the sole effect of the wettability. No hysteresis in the boiling curve was observed for both cases. The ONB was observed after 3.5 °C superheat on the hydrophobic case and the heat transfer coefficient (HTC) changed suddenly from the one of a convection regime (1.5 kW/m² K) to the one of a nucleate boiling regime (4 kW/m² K). On the contrary for the hydrophilic case, despite superheat above 37 °C and presence of boiling, the HTC was kept as the one of the convection regime.     

Comparative analysis of thermodynamic performance of CO2 cascade refrigeration system assisted with expander and mechanical subcooling

In order to discuss the feasibility of using R744/R744 cascade refrigeration system (CRS) instead of R744/R717 CRS, six configurations of R744/R744 CRS assisted with expander and mechanical subcooling system (MS) are analyzed. Based on the thermodynamic analysis, the results show that the high pressure, the condensing temperature of the low‐temperature cycle (LTC), and the degree of subcooling of LTC and the high‐temperature cycle (HTC) are three important operating parameters with an optimum value corresponding to the maximum coefficient of performance (COP). Compared with other CRSs, CRS with HTC throttling valve and MS of HTC (CTS_H) and CRS with HTC expander and MS of HTC (CES_H) show an excellent performance. CES_H has the highest COP and is improved by an average of 13.8% compared with the COP of R744/R717 CRS. The COP of CTS_H is improved by an average of 4.2% compared with the COP of R744/R717 CRS. In conclusion, it is an efficient way to improve performance that CRS combines MS in HTC and HTC expander. And it is possible R744/R744 CRS instead of R744/R717 CRS.