Heat Transfer Coefficient Research Articles

Role of silver nanoparticle in thermal energy process regulated by peristalsis

Electroosmosis regulated model of peristalsis with nanoparticles is significant for heat transfer efficiency regarding optimization of thermal energy process. Such investigation is useful for processes in nano-drug delivery systems, pharmacology, renewable energy, energy saving, drying, biochip fabrication, Sensing and imagining, hyperthermia, cryosurgery, oil mobility improvement, filtration and purification, drilling, cell separation and microelectro-mechanical Systems (MEMS). Therefore, mixed convective peristaltic flow of nanomaterial filling porous medium is addressed. Asymmetric flow configuration is taken. Nanomaterial comprising silver and water is considered. Thermal radiation and dissipation are invoked. Peristaltic pumping is studied under the process of electroosmosis. Electroosmosis process is modeled by Nernst-Planck and Poisson equations. Debye-Huckel assumption is used for electric potential distribution along electron double layer. Velocity slip and convective boundary constraints are considered in this analysis. Dimensionless equations are presented employing lubrication approach. Related problems have been computed numerically. Behaviors of temperature, velocity, pressure gradient and streamlines are examined graphically. Heat transfer coefficient is also studied. Result shows that temperature decreases by volume fraction of nanoparticle. Pressure gradient decays for larger porosity. Temperature reduces for larger radiation. Rate of heat transfer enhances by Grashof number whereas it reduces by volume fraction of nanoparticles.

Temperature prediction of geothermal drilling considering the drilling fluid thermophysical parameters

An increasing number of geothermal wells are being drilled to meet the growing demand for geothermal energy. Given that the harsh environment of geothermal wells exposes drill-following tools to the risk of high-temperature failure, scholars have developed thermo-mathematical models to predict the temperature field distribution in wellbore. To the best of authors' knowledge, most models ignore the interaction between the fluid thermophysical parameters and its temperature. They also overlook the fact that the actual structure of the drilling column causes changes in the convective heat transfer coefficients between the fluid and the well wall, which subsequently affects heat transfer from the wellbore. In this study, we tested the changes of thermal property parameters of drilling fluid with density and temperature. Based on the actual drill string structure and considering the variation in thermophysical parameters during drilling fluids’ circulation, we innovatively established a transient heat transfer model for geothermal horizontal wells with heat–fluid–solid coupling. Subsequently, a drilled geothermal well was used to simulate the temperature during drilling. The effects of the drilling fluid system, density, and displacement, rotary speed, and WOB during drilling on the wellbore temperature distribution were analyzed. Furthermore, suggestions for the selection of drilling fluid and parameters during the drilling process were given. The results of this study can help accurately predict the wellbore temperature of geothermal wells and high-temperature wells, which provides guidance for the rational selection of drilling tools for drilling operations.

Experimental and numerical investigation of the thermal - hydraulic performance of flow inside helical coil elliptic tubes

Helical coil tubes are essential in a variety of engineering applications due to their improved heat transfer and compact shape. The current study presents an experimental and numerical investigation of geometrical optimization for helical coil elliptic tubes. Four distinct coil pitch values are examined with a constant perimeter for all geometries and three aspect ratio values (1, 0.75, 0.5) utilizing water as the working fluid across varying the volume flow rates. The helical coil has the same surface area as the tube for each coil. The numerical simulation is conducted using the ANSYS FLUENT CFD software (Version 19.2). The experimental and numerical results indicate that the increase in heat transfer due to an increase in mass flow rate is significantly greater than the improvement gained by changing the aspect ratios. Increasing the volume flow rate to 3 L/min significantly improves the average heat transfer coefficient, with a 66.6 % increase found at a maximum coil pitch of 0.08 m and an aspect ratio of 0.5. Furthermore, a helical coil with an aspect ratio of one (circular cross-section) exhibits the best compromise between pressure drop penalty and heat transfer enhancement, resulting in the greatest hydraulic thermal performance parameter.

Investigation of performance and emission values of new type of fuels obtained by adding MgO nanoparticles to biodiesel fuels produced from waste sunflower and cotton oil

Biodiesel was produced using the transesterification method from waste sunflower and cotton oil. As a result of the analysis of the new type of nanoparticle-added biodiesel fuels we produce, the parameters with a decrease in emissions are CO, HC, and smoke emissions. There is a partial increase in NOx emissions. In addition, nanoparticle addition made a positive contribution by increasing thermal efficiency. The produced MgO nanoparticle-doped biodiesel fuel samples were compared with diesel fuel. As seen in these comparisons, nanoparticle additives contributed positively to reducing emission values. These improvements were observed as a 31.25 % reduction in CO in the WSOB5+ 100 ppm MgO fuel sample, 41.6 % reduction in HC in the COB5+ 100 ppm MgO fuel sample and 36.92 % reduction in smoke (soot) in COB5+ 100 ppm MgO fuel sample. A comparison was made between the fuel samples in our study, biodiesel produced from waste sunflower and cotton, and fuels produced with nanoparticle additives of 50, 75, and 100 ppm. It was observed that the most efficient fuel sample in terms of emission values and performance was COB5 + 100 ppm MgO nanoparticle additive fuel. The biodiesel fuel sample produced with COB5 + 100 ppm MgO nanoparticle additives provided a 3.25 % improvement in thermal efficiency compared to biodiesel without additives. A positive effect on the heat transfer coefficient was observed as a result of the data on the new type of nanoparticle doped biodiesel. With the impact of this contribution, parameters such as positive contribution to in-cylinder temperature, ignition delay, and combustion time were positively affected in the same direction. Another positive effect of nanoparticle additive to biodiesel, thermal efficiency, increased by 1.1 % in the COB5 + 100 ppm MgO fuel sample. Depending on the decrease in emission rate, it was concluded that the nanoparticle additive added to the fuel greatly benefits the environment.

Experimental study of subcooled flow boiling in microchannel heat sinks integrated with different embedded pin fin arrays microstructures

The optimized microchannel heat sinks could enhance flow boiling for effectively tackling the electronics cooling. The flow boiling experiment for three microchannel heat sinks integrated with different layouts of entrenched pin fins is conducted at flow rate of 273.6–456 kg/(m2.s) and inlet subcooling of 35∼50 K. The overall/local heat transfer features, pressure drop and boiling mechanism are studied. The hybrid pattern presents earliest initial boiling and lower superheat than the microchannel heat sink with uniform pin fins arrangement at moderate and large flow rates. The trend of overall and local HTC (heat transfer coefficient) is similar, which occurs peak at onset nucleation boiling, and then decreasing with increasing heat flux. At the largest flow rate, the hybrid pattern exhibits 2.7–3.5 times peak HTC promotion than other patterns. As for lowest flow rate, the hybrid pattern does not manifest remarkably superior performance due to downstream vapor cores clogging effect. The hybrid pattern shows largest pressure drop, and the smaller inlet subcooling manifests inferior heat transfer and resistance performance. The comprehensive performance factor (CPF) is proposed, and the pattern with uniform small-sized pin fins shows optimal CPF especially for low flow rate, which is considerable compared with the reference heat sink structures until high heat flux. This study may provide some insight into the design of microchannel for flow boiling heat dissipation.

Effect of an external axial magnetic field on thermal convection of an electric conducting fluid in a rotating spherical shell

We investigated the interaction between the thermal convection structure in a rotating spherical shell and a uniformly applied magnetic field acting externally toward the axis of rotation. We considered a system that the inner and outer spherical boundaries are heated and cooled both isothermally and the effects of induced magnetic fields are neglected. The convection structure of this system is characterized by the generation of travelling waves around the axis of rotation. We numerically investigated how this is affected under the magnetic field. The characteristics of the disturbance (i.e., growth rate and phase velocity) were estimated from the exponential amplification period using three-dimensional (3D) simulations and compared with those obtained from linear stability analyses (LSA). In particular, the effects of the magnetic field on the kinetic energy and Nusselt number were investigated for a quasi-steady state using 3D simulations. Our numerical results revealed that the applied magnetic field destabilizes the flow field and enhances heat transfer. This is noteworthy, contrary to the common sense that magnetic field stabilizes the flow field and reduces the heat transfer coefficient. However, the magnetic field strengths at which the kinetic energy and heat transfer coefficient took their maximum values differ slightly.

Numerical simulation of fluidized bed reactor for calcium looping energy release process in thermochemical storage: Influence of key conditions

The thermochemical energy storage technology applied to concentrating solar power is expected to realize the large-scale deployment of solar power. Reactor design is recognized as a key challenge. In this study, the calcium looping energy release process in a bubbling fluidized bed reactor was numerically simulated using an Eulerian-Eulerian Two-fluid Model. The effects of immersed tubes, particle size, CO2 concentration, and flow velocity on the multi-physical coupling characteristics during the carbonation process were explored. The result indicated that the presence of immersed tubes improves flow characteristics and effectively controls the bed temperature. Particle size had no significant effect on the carbonation process during the kinetically controlled stage. Increasing CO2 concentration significantly enhanced the conversion rate; however, it could lead to a reduction in particle renewal frequency, thereby deteriorating the heat transfer coefficient (HTC). The study on flow velocity indicated that a balance should be struck between bed stability and reaction conversion rate, with the model identifying 0.48 m/s to 0.74 m/s as the suitable operating gas velocity. Additionally, the study compared two methods for calculating HTC and analyzed the energy utilization efficiency during the carbonation process, providing recommendations for industrial applications.

Experimental and modeling study of the interfacial and convective heat transfer coefficients of 6061 aluminum alloy in hot gas forming

Experimental and modeling study of the interfacial and convective heat transfer coefficients of 6061 aluminum alloy in hot gas forming

Numerical investigation of the effect of the structure and number of spiral baffles on the vibration and heat transfer coefficient of spiral tubes within spiral tube heat exchangers

In response to the advantages of compactness and high efficiency of spiral elastic tube heat exchanger and its easy-to-vibrate characteristics, this study aims to further improve its heat transfer efficiency by utilizing fluid-induced vibration-enhanced heat transfer technology, so a design scheme of reverse baffle spiral tube heat exchanger (RB-STHE) is proposed to enhance the turbulence characteristics of the shell-side fluid by installing spiral baffles, thus promoting the vibration of the spiral tube. In this study, four new types of the RB-STHE (Model A, B, C, and D) were used as research objects, and the two-way fluid–structure coupling calculation method (using CFX and Transient Structure software) was adopted to systematically investigate the effect of fluid-induced spiral tube vibration on the comprehensive heat transfer coefficient (HTC) of the RB-STHE under different flow velocities and structure schemes, and based on the research data, the objective of no reduction in the comprehensive HTC and savings in the use of materials is taken as the goal, so as to put forward the improved Model C-1. The results show that: although the fluid-induced spiral tube vibration can effectively improve the HTC of the spiral tube, it also increases the pressure drop in the RB-STHE, and the larger the amplitude, the higher the degree of influence; among them, Model C is most affected by the spiral tube vibration, and its HTC and pressure drop are maximally enhanced by 6.33% and 5.85%, respectively. The change in the number of baffles has a significant effect on the comprehensive HTC of the RB-STHE. Compared to the Model D without a spiral baffle, the maximum improvement in the comprehensive HTC of Model C with one spiral baffle installed in the RB-STHE is 14%, while the Model A with four spiral baffles and the Model B with two spiral baffles have a maximum reduction in the comprehensive HTC of 20% and 3%, respectively. In addition, compared to Model C, the structure improved Model C-1 significantly reduces the size of the spiral baffle while effectively maintaining the comprehensive HTC of the RB-STHE, making the structure more reasonable, which provides a reference for heat exchanger design.

Heat transfer performance and startup characteristics of separated gravity heat pipe

Separated gravity heat pipes, with distinct arrangements of evaporating and condensing sections, offer low thermal resistance, high heat transfer efficiency, and a simple structure, making them suitable for passive cooling of spent fuel pools through gravity-driven processes. In this study, a steady-state, one-dimensional, two-phase flow model is developed to analyze phase change heat transfer in a separated gravity heat pipe. The research investigates the impact of liquid filling rate and vacuum level on heat pipe operation. The findings reveal a notable increase in pressure drop inside the tube with higher liquid filling rates, which diminishes the adaptability of the heat pipe as pressure rises, resulting in a narrower range of effective liquid filling rates. Specifically, the liquid filling rate of the heat pipe ranges from 38 % to 72 % at P = 13.75 kPa and from 44 % to 68 % at P = 15.75 kPa. Additionally, a startup prediction model is formulated based on a thermal resistance network model using the ideal gas assumption. Two startup states—namely, the initial startup state and the subsequent startup state—are proposed for large-scale separated gravity heat pipe arrays. The heat pipe achieves the large-temperature-difference initial startup state at a 50 % liquid filling rate within 7 min before transitioning to the subsequent startup state. The vapor mass, heat transfer coefficient, and working fluid temperature initially increase and then decrease over time, with varying peak times. As the liquid filling rate increases, the maximum vapor mass also increases. The heat transfer coefficient peaks at a 60 % liquid filling rate before subsequently declining. The optimal liquid filling rate of 60 % is identified based on startup time, heat transfer coefficient, and overall heat transfer efficiency.

Assessment of a mixed-mode vertical solar dryer for experimental turmeric drying

Solar drying, an eco-friendly and sustainable approach harnesses solar radiations to remove moisture from a drying commodity, thereby preventing its spoilage and extending shelf life. Turmeric (Curcuma longa), a popular herb is being used for centuries in Ayurveda as a traditional medicine. Since, fresh turmeric contains 80–90 % moisture content, therefore, it is quite difficult to store for a longer period. An innovative and compact mixed-mode vertical solar dryer (MVSD) with a circular solar collector (solar tracking geometry) is developed and tested for turmeric slices drying under partial and fully loaded conditions. Turmeric samples of 2, 3 and 4 mm slices thicknesses were used for experimentation under partial loading condition. The thickness of the turmeric slices was observed to affect the performance of the MVSD. The drying rate and thermal efficiency were observed to be highest for 3 mm thick turmeric slices sample under partially loaded condition. The MVSD was further tested with full loading capacity for 3 mm thick turmeric slices drying under natural and forced convection (NC and FC), respectively modes. The drying behavior of turmeric slices was well described by Midilii-Kucuk model. Heat transfer coefficients and efficiency (thermal and exergy) were observed to be higher under FC mode. Embodied energy and energy payback time were observed as 554.44 and 585.53 kWh; and 4.39 and 4.55 years under NC and FC modes, respectively. Annualised cost and payback period were observed to be desirable for NC mode.

Experimental investigation of pressure drop oscillation and active mitigation in a pumped two-phase loop

A pumped two-phase loop (P2PL) integrated with a microchannel evaporator is the most favored design for advanced thermal management of modern electronic devices. However, these systems are susceptible to two-phase flow instabilities, in particular pressure drop oscillation (PDO), which can compromise their performance by premature initiation of critical heat flux (CHF), deterioration of the heat transfer capabilities, and inducing severe thermal and pressure fluctuations. This study experimentally investigates PDO in a P2PL with a microchannel evaporator, exploring the underlying mechanisms and mitigation strategies. The range of parameters are: heat flux from 0 to 56 W/cm2, mass flux from 47.9 to 95.9 kg/m2-s, a fixed inlet temperature of 10 °C, initial accumulator gas pressure from 620.5 to 827.3 kPa, and vapor quality from zero (subcooled liquid) to one (pure vapor close to dryout). A consistent trend of high-amplitude, low-frequency PDO at low heat fluxes transitioning to low-amplitude, high-frequency PDO at high heat fluxes was observed across all operating conditions. PDO originating in the evaporator propagated to other loop components with similar characteristics, underscoring the systemic nature of PDO. Further, PDO-driven thermal oscillations were observed to propagate through the evaporator, reaching the heater (heat source) temperature with maximum amplitude of 3.4 °C. These oscillations had a detrimental impact on the heat transfer coefficient by altering the flow boiling regimes within the microchannels. Notably, the subcooled liquid temperatures remained stable, indicating the absence of pressure-induced thermal effects in single-phase regions. Finally, an active control technique, based on controlling the pump flow rate using continuous feedback of flow rate was implemented to mitigate PDO, eliminating both pressure and temperature oscillations, achieving an average reduction of 69.8 % in PDO amplitude.

Enhancement of pool boiling under reduced gravity by the synergistic effect of non-uniform surface structure and electric field: A lattice Boltzmann study

In this paper, a novel non-uniformly structured surface with primary and secondary pillars is conceived to be coupled with the action of an electric field to enhance pool boiling under reduced gravity. On the surface, the secondary pillars are distributed diagonally. A three-dimensional thermal lattice Boltzmann (LB) model combined with an electric field model is employed to investigate the boiling heat transfer performance on the non-uniformly pillar-structured surface and the associated boiling enhancement mechanism. It is found that the synergistic effect of the non-uniform surface structure and the electric field leads to a non-equilibrium electric force that causes the bubbles to be pushed away from the center of the heating surface, which promotes an earlier departure of the bubbles and provides more paths for the fresh liquid replenishment. As a result, the boiling performance under reduced gravity can be significantly enhanced by utilizing such a synergistic effect, with both the critical heat flux (CHF) and the maximum heat transfer coefficient (HTC) on the non-uniformly pillar-structured surface exceeding those on a uniformly pillar-structured surface under normal gravity. Moreover, the influences of the distribution of the secondary pillars on the boiling performance under reduced gravity are also studied. It is shown that the non-uniformly pillar-structured surface whose secondary pillars are distributed diagonally performs much better than those with the secondary pillars being distributed crossly and centrally, and the synergistic effect of the non-uniform surface structure and the electric field results from tuning the distribution of the non-uniform electric force.

Numerical simulation on the condensation heat transfer and flow characteristics outside smooth and enhanced tubes

Most of the current studies only conducts two-dimensional simulations of condensation heat transfer and flow outside enhanced tubes, which cannot accurately reflect the flow and heat transfer characteristics of liquid film outside enhanced tubes. In order to provide theoretical support for the design optimization of enhanced tubes, this study adopts the VOF multiphase model and Lee condensation model to simulate the condensation heat transfer(HTC) of refrigerants outside smooth and enhanced tubes. The instantaneous film flow characteristics of the liquid film outside the horizontal smooth and enhanced tubes are analyzed, and the thickness of the liquid film and the relationship with the condensation heat transfer coefficient of the smooth and enhanced tubes under different working conditions are calculated. The refrigerants are compared under the same working conditions, and the results are shown: The simulation results are in good agreement with the experimental data and the Nusselt analytical solution, and the error is all within 15%; the distribution of liquid film is highly sensitive to the magnitude of local condensation heat transfer coefficients, and the condensation HTC outside the enhanced tube is approximately six times of that of smooth tube. Combining the distribution of the liquid film and the thermophysical properties of the refrigerants, it was determined that R410A has the highest HTC, while R1234yf has the lowest HTC.

Experimental investigation on flow boiling heat transfer of R290, R600a and their mixtures in a horizontal minichannel

The flow boiling heat transfer characteristics of R290, R600a and their mixtures were experimentally investigated in a horizontal minichannel with an inner diameter of 1.54 mm. In order to investigate the effects of mass flux and vapor quality, heat flux, saturation temperature, and component concentration on the flow boiling heat transfer coefficients, experiments were conducted with mass fluxes ranging from 200 to 400 kg/(m2·s), heat fluxes ranging from 20 to 30 kW/m2, saturation temperatures of 22 °C, 24 °C, and 26 °C, vapor qualities ranging from 0 to 1, and mass ratios of 70/30, 50/50, and 30/70 for R290/R600a. The experimental results indicate that with the increase of vapor quality, the mass flux promotes the heat transfer coefficient more obviously, and the heat transfer coefficient tends to increase first and then decrease. The heat transfer coefficients of both pure and mixed refrigerants increase with increasing the heat flux. The effect of saturation temperature on the heat transfer coefficients of pure and mixed refrigerants is not significant in this study. In the low vapor quality region, the heat transfer coefficients of R290/R600a mixtures are between those of R290 and R600a. As vapor quality increases, the heat transfer coefficients of the mixtures increase more slowly than those of pure refrigerants. In addition, experimental data were compared with the predictions of the empirical correlations. Among all the correlations, the Tran et al. correlation predicts the heat transfer coefficients for R290 and R600a best. The Lim and Kim correlation gives the best predictions of the heat transfer coefficients for R290/R600a mixtures.

Modeling of bed-to-wall heat transfer coefficient in fluidized adsorption bed by gene expression programming approach

Adsorption cooling and desalination methods with adsorption chillers (AC) are promising in energy technologies. However, the low-performance coefficient and bulkiness of traditional packed-bed ACs, primarily due to the high voidage of the sorbent beds leading to low heat transfer coefficients, pose significant challenges. Despite numerous attempts, a practical solution to this problem is yet to be found.In response to this challenge, we propose a novel approach: a fluidized adsorbent bed instead of the traditional packed bed. We also introduce gene expression programming (GEP) as an innovative artificial intelligence (AI) method for modeling the bed-to-wall heat transfer coefficient in the adsorption bed. Our study includes calculations and model validation for a heat transfer adsorption bed reactor designed for low-pressure adsorption processes. The fluidizing agent of the adsorbent bed was water vapor generated in the evaporator. Silica gel was used as the parent adsorption material in our tests. The heat transfer coefficient was successfully validated and determined through experiments and estimated using the formulated (b-t-wHTc) meta-model. The data evaluated by the model aligns well with the experimental results. Our calculations demonstrate that the GEP-based model accurately predicts the heat transfer coefficient and is suitable for analyzing the fluidized adsorption bed reactor.The outlined studies serve as a benchmark for subsequent simulations of the intensified heat transfer adsorption bed reactor, as they are integral to project No. 2018/29/B/ST8/00442, titled “Research on sorption process intensification methods in modified construction of adsorbent beds,” supported by the National Science Center in Poland.

Enhanced Phase Change Heat Transfer with Fused Deposition Modeling (FDM) Printed Pit and Pillar (Pi2) Arrays

Phase change heat transfer, crucial in thermal management systems, can be significantly enhanced through optimized surface structures. This study investigates pool boiling heat transfer enhancement using 3D printed structures with carefully designed pillar and pit geometries. We present a novel approach combining the Dual Rise model with separate liquid–vapor pathways to improve Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC). Using copper-infused Polylactic Acid (PLA) filaments, we created and sintered structured surfaces featuring pit-assisted nucleation sites, interpillar spacing for vapor escape, and pillar roughness for enhanced liquid supply. Experiments with deionized water and ethanol under atmospheric pressure demonstrated substantial improvements over plain surfaces: water showed an 87% increase in CHF and 39% in maximum HTC, while ethanol exhibited even greater enhancements of 122% in CHF and 61% in HTC. These improvements are attributed to the synergistic effects of optimized surface geometry and separated liquid–vapor pathways, reducing counterflow resistance and improving hydrodynamic stability. A theoretical framework based on the Dual Rise model explains these enhancements, providing insights into coupled capillary action and hemiwicking effects in boiling heat transfer. The study introduces predictive models for CHF and HTC enhancement, offering valuable tools for future design optimization in applications ranging from electronics cooling to power plant thermal management and advanced heat exchangers.

Record-high heat transfer performance of spray cooling on 3D-printed hierarchical micro/nano-structured surface

Managing high-flux waste heat with controllable device working temperature is becoming challenging and critical for the artificial intelligence, communications, electric vehicles, defense and aerospace sectors. Spray cooling, which combines forced convection with phase-change latent heat of working fluids, is promising for high flux heat dissipation. Most of the previous studies on spray cooling enhancement adopted high spray flow rates to strengthen forced convection for high critical heat flux (CHF), leading to a low heat transfer coefficient (HTC). Micro/nanostructured surfaces can enhance boiling, but bubbles inside the structures tend to form a vapor blanket, which can deteriorate heat transfer. This work demonstrates simultaneous enhancement of CHF and HTC in spray cooling by improving both evaporation and liquid film boiling on three-dimensional (3D) ordered hierarchical micro/nano-structured surface. The hierarchical micro/nanostructured surface is designed to coordinate the transport of spray droplets, capillary liquid films, and boiling bubbles to enhance spray cooling performance. Boiling inversion where superheat decreases with increasing heat flux is observed, leading to an ultra-high HTC due to the simultaneous promotion of bubble nucleation and evaporation. Unprecedented CHF is obtained by overcoming the liquid-vapor counterflow, i.e., synergistically facilitating bubble escape and liquid permeation. A record-breaking heat transfer performance of spray cooling is achieved with a maximum heat flux of 1273 W/cm2 and an HTC of 443.7 kW/(m2 K) over a 1 cm2 heating area.

The influence of longitudinal duct profiling on unsteady gas dynamics and the heat transfer of pulsating gas flows in the outlet system of reciprocating-engine

Heat machines based on reciprocating-engines remain in demand in various fields of engineering and technology. Therefore, further research is needed to improve the efficiency, reliability, and environmental friendliness of engines. The thermomechanical improvement of non-stationary processes in outlet systems is an appropriate way to improve engine performance. The purpose of this research is to obtain and analyse the gas-dynamic and heat transfer characteristics of pulsating gas flows in an outlet system with ducts of various designs, using a laboratory model to simulate the outlet process in an engine. Thermal anemometry is used to receive data on the instant velocity values and local heat transfer coefficient of unsteady flows in ducts. The article examines two designs of outlet ducts, namely cylindrical (basic) and conical (with a taper of 0.0225) ducts. Spectral analysis of velocity, pressure and heat transfer coefficient pulsations, assessment of turbulence intensity, and calculation of flow characteristics of pulsating flows were performed to obtain detailed information on gas dynamics in the outlet system. The use of a conical duct (in comparison with a cylindrical one) leads to a slight increase in the turbulence intensity by up to 12 %, a decrease in the heat transfer coefficient by 15–20 %, and a change in volumetric gas flow within ± 7.5 %. Thus, the use of a conical duct will lead to the stabilisation of the flow in the outlet system, improved cleaning of the cylinder from outlet gases, a reduction in thermal stress, and a slight growth in the specific power of engines.

New quantitative characterizations of flow and heat transfer in hot dry rock fracture

The overall heat transfer coefficient (OHTC) of rock fractures is a fundamental parameter for characterizing the heat transfer behavior of rock fractures in hot dry rock (HDR) geothermal mining. Although a number of practical formulas for heat transfer coefficients have been developed in the literature, there is still no widely accepted analytical solution. This paper constructs highly accurate analytical solutions for the temperatures of the inner fracture wall and the fluid. Then they are employed to develop new definition-based formulas (formula A and its simplification formula B) of the OHTC, which are well validated by the experimental and numerical simulation results. An empirical correlation formula of heat transfer coefficient is proposed based on the definition-based formulas which can be directly used in the numerical simulations of heat transfer in rock fractures. A site-scale application example of numerical simulation also demonstrates the effectiveness of the empirical correlation formula.