Heat and Mass Transfer最新文献

Silver nanofluids have recently emerged as a promising coolant for enhancing heat transfer performance. This study experimentally investigates the heat transfer characteristics of silver nanofluid in a helical shell and tube heat exchanger. Spherical silver nanoparticles of 143 nm mean diameter were synthesized using a chemical reduction method and characterized comprehensively. The nanofluid was utilized as the tube-side coolant at volumetric concentrations of 1.5% and 2.5%, with its effectiveness compared to water. The impacts of concentration and fluid flow rate on heat transfer coefficient and effectiveness were evaluated under varying conditions. Results showed improved heat transfer performance using silver nanofluid, with the maximum enhancement at 2.5% concentration. The heat transfer coefficient and effectiveness increased with higher flow rate, demonstrating the importance of optimizing fluid flow conditions. This study provides new insights into harnessing silver nanofluids for thermal engineering applications and quantifies the effects of concentration and flow on the viability of silver nanofluids as efficient coolants in heat exchangers.

This work aims to use experimental data from thermal characterization and adsorption/desorption isotherms of two tropicals woods species (Ayous and Tali) to propose an empirical model of thermal conductivity as a function of air relative humidity. A static gravimetric method was used to determine the adsorption isotherms of Tali and Ayous at 30 °C, and 40 °C. The GAB, Henderson and Nelson models were used to predict the isotherms. Exponential models of thermal conductivity and volumetric heat capacity with air relative humidity were proposed. The influence of hysteresis phenomenum was studied on these properties. The reliability of the developed empirical correlation between thermal properties and air relative humidity was evaluated by comparing the experimental and predicted curves. The relative errors were less than 8% for both Ayous and Tali. The correlation coefficients obtained were greater than 99% for both species in adsorption and desorption. There was also an increase in the equilibrium water content of both species with the increase in water activity at constant temperature. The correlation coefficients between GAB model and sorption experimental data were lower than 99% when Ayous was subjected to a temperature of 40 °C in adsorption and Tali to a temperature of 40 °C in desorption.

A potential solution to environmental problems associated to the use of fossil fuels and the exploitation of natural resources for energy production is the development of renewable energies with greater capacity, adaptability and integration, as well as their use for the improvement of this systems. Researchers turned their attention to biological and natural processes such as honeycombs at a structural level to increase the mechanical properties of various technologies. This investigation shows the results of the thermal analysis of a novel solar collector designed based on a Honey-Comb conjecture studied under different connections. Several structures were proposed considering a serial and parallel connections. Each one was designed and simulated in SolidWorks^® software Flow Simulation. The study considers different boundary conditions as mass flow and solar radiation on the surface of the collectors. In the analysis, the maximum temperature was achieved at the highest solar radiation of 1050 (W/{m}^{2}) and the lowest flow mass of 0.052 kg/s. On the other hand, the peak performance of the heat thermal parameter in the whole study was achieved at solar radiation of 1050 (W/{m}^{2}) and the maximum mass flow of 0.17 kg/s. A honey-comb structure conformed by three collectors (AC1) shows an increase of around 187%, against a single collector (A0), comparing the other structures two collectors in series (AS1) and two collectors in parallel (AP1) connections the total increase in the useful heat obtained with AC1 was 52% and 49% respectively.

One of the limitations to design a compact heat exchanger is the phenomenon of fluid maldistribution. Most research considers a uniform fluid distribution in the channels, which can be considered a wrong approximation, since the non-uniform fluid distribution can seriously affect the performance. There are few experimental and numerical studies related to fluid distribution in a compact heat exchanger, however, currently, there is no mathematical model capable of predicting the fluid distribution within the channels. The present work developed the first theoretical model capable to estimate the flow distribution inside compact heat exchanger channels. The model is based on the concept of the shape factor, relating the radiation ratio between surfaces with the mass flow rate. The model considers geometric parameters to estimate the fluid distribution, such as channel position, channel cross-sectional area, fluid inlet surface area, and inlet header depth. In order to verify the model's accuracy, comparisons with experimental and numerical data available in the literature were performed, besides, a test facility was produced and used to test two header configurations. The average error of the model was approximately 9%, having a better performance than the hypothesis of uniform distribution, which presented an average error of 13%. However, in cases where fluid maldistribution was pronounced, the model exhibited significantly better results, reducing the error from 29%, uniform distribution hypothesis, to 11%. This demonstrates that the model can be applied to estimate fluid distribution inside de core and enhance the design of heat exchangers.

Considering that methane hydrate formation in the water drainage line of natural gas hydrate production well could induces a series production issue, the experiments of methane hydrate equilibrium and formation in the water-methane-silt sand system are conducted to reveal the effect of silt sand on methane hydrate equilibrium and formation by using the low-temperature and high-pressure rocking reactors. In experiments, the silt sand mass concentrations are from 0 to 4_wt% and the initial system pressures are from 4.38 to 12.14 MPa. The results from methane hydrate equilibrium experiments indicated that the silt sand could move the methane hydrate equilibrium curve to right about 1 °C, which a favorable environment for methane hydrate crystallization is created by silt sand. The correlations of methane hydrate equilibrium considering the effect of silt sand is obtained empirically. Moreover, the results from methane hydrate formation experiments showed that the silt sand could enhanced methane hydrate formation rates and promoted the water conversion ratios. The higher initial pressure conditions result in higher hydrate formation rates. The influencing mechanism of silt sand on the methane hydrate equilibrium and formation is considered to be similar with the nanofluid, which silt sand aggregate the heat and mass transfer process during methane hydrate crystallization.

Based on the analysis of existing studies on the calculation of the process of compression of a droplet liquid in a displacement pump, we developed a method for assessing the effect of external heat transfer, deformation work and mixing heat transfer on the working fluid heating in the pump. Using the results of a numerical experiment on the increase in pressure and temperature during compression in a positive displacement pump, it was found that the greatest influence on the increase in pressure during compression is by deformation processes (an increase in pressure due to a change in volume ranges from 80 to 92%), then there is mass transfer (pressure increase is from 7 to 16%) and heat exchange, the values of which are about 2.5%. The decisive effect on the working fluid heating in the working chamber of the pump is the conversion of deformation work into heat (from 92 to 95%), the values of external and mixing heat transfer are approximately the same and range from 2.5% to 3.5% each. The nature of the effect of the independent variables used (discharge pressure, crankshaft speed, radial clearance in the cylinder-piston group and the average temperature of the working chamber surface) on each of the components of the relative change in pressure and temperature during the compression process has been established. We established that the crankshaft revolutions has the greatest effect on the relative increase in pressure and temperature during compression, followed by the value of the radial clearance and discharge pressure. The average temperature of the surface of the working chamber has practically no effect on the increase in pressure due to the processes of deformation, mass transfer and heat interaction and only affects the relative change in temperature due to external heat transfer.

Conventional heat pipes with one evaporator and one condenser are used to cool only one heat source at a time. In electronics and space applications, where a large number of heat sources are to be cooled with limited space available, a multi branch heat pipe could be the solution. In the present study, a heat pipe (T-shape) with three branches was developed with 20 number of axial grooves in which two branches worked as evaporators and one branch as a condenser. Experimental study was performed by considering four novel types of orientations i.e. (a) horizontal orientation (HO) (b) gravity assisted orientation (GAO) (c) anti-gravity orientation (AGO) and (d) compound orientation (CO). The results are analyzed in terms of start-up characteristics and total heat transfer coefficient at different heat loads. Evaporator and condenser thermal resistances are calculated and analyzed for better understanding. It was found that horizontal orientation resulted in the highest overall heat transfer coefficient (2.72 kW/m² ℃ at 240 W) and comparatively lower evaporator temperatures (less than 100 ℃ at 240 W) which is suitable condition for electronics cooling. Maximum effective thermal conductivity of 31.82 kW/m ℃ was achieved in horizontal orientation. It also resulted in lowest evaporator resistance (0.157 ℃/W) and lowest condenser resistance (0.114 ℃/W). Phenomena of temperature jump was observed and elaborated for compound orientation.

The surface roughness features that develop on a three-dimensional (3D) carbon/carbon (C/C) composite during ablation, that is, material loss and morphology distribution on the wall, were investigated, and a microstructure model was established to analyze the flow field characteristics on the C/C composite surface. The model relies on two changes of scale: (i) the multi-wave height (bundle) varies from 50 μm to 110 μm and (ii) the bundle diameter varies from 0.3 mm to 0.5 mm. At each scale, the 2D full Navier–Stokes surface equation was solved numerically to obtain the heat, friction, and pressure in the steady state. Roughness disturbs the flow properties of the boundary layer, creating additional heat flow and aggravating ablation. Numerical results in a hypersonic gas-thermal environment show the distribution characteristics of the coarse-walled heat flow. Thermochemical ablation preserves the roughness profile and wavefront, which changes the distribution of the external flow field. The flow-heat-ablation analogy study can effectively characterize the flow-field distribution characteristics and timely heat and mass transfer responses of materials under rough walls. Innovative microstructure simulation showcases the intrinsic relationship between microstructure roughness, ablativity, and thermal mechanical properties. These intrinsic laws and data can make significant contributions to the design and optimization of thermal protection systems.

In this paper, a multilayer perceptron (MLP)-type artificial neural network model with a back-propagation training algorithm is utilized to model the bubble growth and bubble dynamics parameters in nucleate boiling with a non-uniform electric field. The influences of the electric field on different parameters that describe bubble’s behaviors including bubble waiting time, bubble departure frequency, bubble growth time, and bubble departure diameter are considered. This study models single bubble dynamic behaviors of R113 created on a heater in an inconsistent electric field by utilizing a MLP neural network optimized by four different swarm-based optimization algorithms, namely: Salp Swarm Algorithm (SSA), Grey Wolf Optimizer (GWO), Artificial Bee Colony (ABC) algorithm, and Particle Swarm Optimization (PSO). For evaluating the model effectiveness, the MSE value (Mean-Square Error) of the artificial neural network model with various optimization algorithms is measured and compared. The results suggest that the optimal networks in the two-hidden layer and three-hidden layer models for the bubble departure diameter improve MSE by 33.85% and 35.27%, respectively, when compared with the best response in the one-hidden layer model. Additionally, for bubble growth time, the networks with two hidden layers and three hidden layers have the 44.51% and 45.85% reduction in error, when compared with the network with one hidden layer, respectively. For the departure frequency, the error reduction in the two-layer and three-layer networks is 46.85% and 62.32%, respectively. For bubble waiting time, the best networks in the two hidden-layer and three hidden-layer models improve MSE by 52.44% and 62.27% compared with the best 1HL model response, respectively. Also, the two algorithms of SSA and GWO are able to compete well (comparable MSE) with the PSO and ABC algorithms.

Miniaturization and enhanced performance of microchips has resulted in powerful electronic devices with high heat flux components. For these advanced electronics, the current heat transfer method of single-phase forced convection is reaching its thermal limit and more effective cooling solutions are needed. A pumped two-phase loop, in which a pump circulates a working fluid that evaporates to absorb heat, can offer a solution. In this paper the cooling performance of a pumped two-phase loop is discussed and validated. A numerical tool has been developed to aid in designing a fit-for-purpose pumped two-phase loop and to predict its behaviour to changing system parameters and heat inputs. Results from the numerical model are compared with temperature, pressure and flow velocity measurements obtained from a prototype setup. The effects of applying varying heat loads on both a single evaporator and on multiple evaporators simultaneously either in series or in parallel have been investigated. Heat transfer coefficients between 7 and 10 kW/m²K were obtained during the experiments. Model predictions correspond well to the measured performances and findings on the two-phase boiling behaviour are presented. The model is particularly useful for the rapid assessment of the layout of a pumped two-phase loop for high heat flux electronics cooling.