A new clamp-on power ultrasound solution is being developed for tubular heat exchangers (HX). This system features software-controlled ability to guide the cleaning effect, to reach also the most challenging locations. The new solution was evaluated by finite-element simulations, and field tests are being carried out on tubular HX. Preliminary results show that the new solution is capable of fouling prevention. The results suggest good potential also for fouling cleaning. In conclusion, the new software-guided clamp-on power ultrasound solution was proven valuable since it permits on-site fouling mitigation, without process interruptions, and by increasing the physical cleaning cycle of tubular HX. The solution will thus result in significant savings in equipment and maintenance costs as well as decreasing energy and production losses.

This study reports the findings of research into the effect of impinging air jet performance caused by flow-blocking elements situated near the orifice exit. The main goal is to figure out how heat transfer performance can be improved with less pumping power. Three distinct types of orifice plates are used to achieve this result. One of these three lacks flow-blocking components altogether, while the other two have variants on this theme. Each of the two orifice plates with flow-blocking components consists of either (i) a wire screen mesh at the orifice exit plane or (ii) distributed orifice (a number of smaller orifices spaced evenly apart). Experiments are performed to cool a flat plate (1.5 mm in thickness) heated electrically by impinging square jets of air at ambient temperature. Three distinct comparison criteria are used to examine the fluid flow, hydrodynamics, and heat transfer properties of the jets emerging from these orifices. For all three jet configurations, these criteria consist of (i) a constant mass flow rate of the fluid, (ii) a constant pumping power consumption of the jet over the orifice plate, and (iii) a constant Nusselt number on the impingement surface. It is seen that by using flow-blocking components at the orifice exit plane improves the heat transfer performance of an impinging air jet. When comparing the two flow blockage designs (distributed orifice vs. orifice with mesh), the distributed orifice configuration performs better.

We presented a new analytic model of physical gas absorption into a stirred liquid. This model is based on the assumption that the gas absorption/desorption process can be considered as a reversible chemical reaction that occurs at the gas–liquid interface. In addition, in this model, there is no assumption about the distribution of the concentration of the dissolved gas inside the liquid during its intensive stirring. In the framework of the presented model, it is shown that the physical absorption coefficient is the rate constant of the desorption reaction. This quantity is individual for each specific gas–liquid system, it does not depend on concentrations, and its temperature dependence obeys the Arrhenius equation. Experiments were carried out to study the kinetics of physical absorption of methane into stirred water at different temperatures and methane pressures. Experiments were also carried out to study the kinetics of physical absorption of carbon dioxide into stirred water at different temperatures and carbon dioxide pressures. The obtained experimental data fully confirmed the validity of the proposed model. The dependences of the rate constant of the desorption reaction (physical absorption coefficient) on temperature for the methane–water and carbon dioxide–water systems were determined.

The turbulator inserted in a heat exchanger is one of the most important methods of enhancing heat transfer passively. It is known that especially coil wire turbulators increase turbulence intensity and thus heat transfer as a vortex generator. Experimental analysis is required to see how the independent parameters, such as thickness, pitch, length and the flow regime affect the increasing heat transfer with increasing pressure loss. For this purpose, in this study, optimum conditions are investigated by using three well-known experimental analysis methods; (i) optimum parameter levels are determined by Taguchi Experimental Design Method (DoE), (ii) Analysis of Variance method (ANOVA) is used in order to determine the effects of each design parameters on the result, and finally (iii) Grey Relational Analysis is applied to the results of heat transfer and pressure drop to obtain the relationship between the highest heat transfer and the lowest pressure drop. The three pitch ratios (p/d = 0.2, 0.3 and 0.4), three thickness ratios (e/d = 0.033, 0.066 and 0.1), three length ratios (l/d = 5, 10 and 15) of coil wire turbulator and six values of Reynolds number ranging from 30 000 to 80 000 are considered to be the design parameters of the experimental study. The extracted results are plotted by means of Nusselt number (Nu), friction factor (f) and thermal performance factor (TPF) which includes both Nusselt number and friction factor in one formula. The experimental outcome showed that the optimum Nusselt number is achieved under the conditions of Re = 80 000, e/d = 0.1, p/d = 0.4 and l/d = 15. On the other hand, the optimum value for the friction factor is obtained under the conditions of Re = 70 000, e/d = 0.033, p/d = 0.4 and l/d = 5. According to Grey Relational Analysis, the design parameters allowing for multi-performance (highest Nu with lowest f) are; Re = 80 000, e/d = 0.1 p/d = 0.3 and l/d = 5.

Water chemistry plays an important role in fouling kinetics and morphology. This work investigates the influence of zinc cations in potable water, specifically the kinetics of crystallisation and their effect on the fouling layer during the operation of a batch steam generator system and a once-through flow system. The kinetics of precipitation in the batch crystalliser were examined based on the change in concentration of the foulants, while the fouling resistance approach was used in the flow system. In addition, morphological testing was carried out using Scanning Electron Microscopy, Powder X-ray Diffraction, and Energy dispersive X-ray. The findings showed that the precipitation rate of calcium carbonate decreases with the increase in zinc ions until reaching the zinc carbonate supersaturation in the water due to water evaporation. Regarding morphology, co-precipitation of zinc carbonate was observed at high zinc concentrations. As a result, a double effect was observed where zinc both retarded and enhanced fouling over time. The fouling rate in the flow system decreased as the zinc concentration increased. Zinc ions were found to influence the morphology of deposit minerals significantly. Moreover, the surface deposition of zinc salts increased with the solution content of zinc.

The measurement data of single droplet evaporation experiments are often biased due to the extra heat input through the fiber suspension and the presence of thermal radiation in hot environments. This encumbers model validation for heat and mass transfer simulations of liquid droplets. In this paper, a thermal analysis of this measurement layout is presented with a coupled lumped parameter model, considering heat conduction through the suspension. The model was validated by experimental data from the literature and good agreements were found. The thermal analysis focused on fiber material and geometry, and thermal radiation properties. Calculations were performed on a broad range of ambient conditions for liquids with different volatility characteristics. Temporal squared droplet diameter- and temperature-profiles, furthermore, droplet stationary evaporation rate were used to characterize vaporization phenomena. The thermal balance of the droplet is dominated by the convective heat rate from the environment in the early stage of evaporation. The effect of heat conduction through the fiber becomes important at the end of the droplet lifetime when the droplet size is decreased. Temperature sensor suspension may seriously bias droplet temperature due to the larger thermal conductivity compared to quartz fiber. Large droplets in high-temperature environments show significant sensitivity to thermal radiation properties, which should be considered in measurements and model validation.

The present work is an investigation to address the drop-in pressure during fluid flow and subsequent maldistribution. Experimental wavy microchannels with four different inlet header geometries in vertical and horizontal directional configurations were fabricated to analyze the fluid flow performance. De-ionized water was used as fluid with varying mass flow rates and subsequently normalized flow rates and non-uniformity factors were determined. Computational fluid dynamic models were developed to determine the effects of four inlet geometries on the Reynolds number and friction factor. Experimental and computational analysis revealed a 50% reduction in maldistribution for conical frustum and semi-circular inlet geometries with the normalized flow rate between 0.98 and 1.04; 0.99 for lower and 1.001 for higher mass flow rates. Conical frustum inlet geometry exhibited relatively better flow performance at higher mass flow rates with lower friction factors than the semi-circular, rectangular, and triangular header. Correspondingly vertical inlet header configuration yielded higher Reynolds numbers at the same flow rates against horizontal configuration without significant variations in friction factors between the two. In both inlet configurations conical frustum inlet geometry exhibited more than 6% improved flow uniformity than other header geometries with steady thermal cooling rate and has exhibited better overall flow performance among the experimental microchannels.

Saving energy resources requires a continuous improvement of the power equipment. The present study aims to develop new designs of double pipe heat exchanger (DPHE) to improve the heating/cooling processes at the lowest possible pumping power. Therefore, thermal performance investigation of three configurations of DPHE has been carried out. These configurations are circular wavy DPHE (DPHEwavy), plain oval DPHE (DPHEov.), and an oval wavy DPHE (DPHEov.wavy). In addition, the conventional DPHE (DPHEconv.) has been employed as a reference heat exchanger, and a validated CFD approach is adopted to perform the current investigation. The findings reveal that, DPHEov.wavy yields the highest Nusselt number (Nu) which is up to 28% with respect to DPHEconv.. In addition, data of pressure drop (ΔP) of DPHEwavy are found the highest followed by those of DPHEconv., whereas DPHEov. is found to yield the lowest ΔP. Furthermore, thermal performance factor (\(\eta\)) has been considered, and DPHEov. is found to own the highest \(\eta\) of all investigated DPHEs. In conclusion, the oval tubes have shown better heat transfer characteristics with respect to their circular counterparts in general, in particular plain oval DPHE.Graphical abstract

Predicting the cleaning time required to remove a thin layer of soil is a challenging task and subject of current research. One approach to tackle this problem is the decomposition into physical sub-problems which are modelled separately and the subsequent synthesis of these models. In this paper, an existing model for adhesive detachment is extended for the prediction of the cleaning time of cohesively separating soil layers. The extension is based on measurements of the pull-off forces and their correlation to the local water mass fraction. The resulting new model is validated using cleaning experiments with starch in a fully developed channel flow. Furthermore, an inhomogeneous soil distribution and its effect on cleaning results like cleaning time and removal rate is investigated. It is shown that accounting for the local soil distribution in the model leads to a significant improvement of the prediction of the cleaning behaviour.

A vehicle braking system comprises disc brakes and brake pads. During braking, kinetic energy is converted into mechanical energy, which must be dissipated heat. The frictional heat generated at the interface between the brake disc and brake pad can lead to high temperatures and affect the service life of the brake disc. Based on the analysis of the thermal behavior of carbon/ceramic disc brakes under different braking modes, where the heat generated is affected by parameters such as speed and friction coefficient, it can be found by comparing the two braking modes that the temperature peak of the carbon ceramic disc brake occurs after 4 s of braking, and the highest-temperature field is distributed on the contact interface between the carbon ceramic disc and brake pad. Results show that the heat generated in the braking mode with variable deceleration is higher than that generated in the braking mode with uniform deceleration, ranging from approximately 60 \(^\circ \text {C}\) to 110 \(^\circ \text {C}\) higher at different braking speeds.

This study presents an experimental investigation to determine the heat transfer enhancement in a novel heat exchanger known as the "Helicoidal Square-Shaped Heat Exchanger" with and without using a nanofluid. The experiments were performed for the range of Reynolds numbers from 4400 to 8000, using nanofluid (Al2O3-pure water) at the concentrations 0.1, 0.25, and 0.5%. This experimental investigation found that the heat transfer ratio is improved by increasing the nanofluid concentrations and the flow Reynolds number. The highest value of the heat transfer ratio was at Re = 8000, and 0.5% concentration of nanofluids. The corresponding increment in the heat transfer rate was 13.46 %, the heat transfer coefficient augmented by 9.64 %, and the Nusselt number improved by 10.43% compared to the results obtained experimentally with distilled water. The results obtained for the distilled water were verified with the Dittus-Boelter equation and numerical simulation. In addition, all obtained experimental data were compared with the CFD simulation. The use of nanofluids for heat transfer enhancement has a wide range of applications. Therefore, the presented results suggest using Al2O3-water nanofluids to improve the efficiency of many renewable energy plants, including solar and geothermal energy systems.

Thermoelectric generators (TEGs) are devices that produce electricity due to the temperature difference between their cold and hot surfaces. The enhanced heat transfer from the cold surfaces of TEGs can increase their electricity efficiency. In this research, an experiment was arranged to study the direct and indirect contact of cooling water on the cold surfaces of the TEGs series and its results on their electricity efficiency. In this experiment, cooling water was flowing inside an aluminum tube with a rectangular cross-section. By changing the slope of the pipe, mixed convection was created in the cooling water. The results showed that the electrical efficiency of the TEGs was higher in the conditions of indirect contact as well as opposing mixed convection. In one of the results, the maximum electricity generation in indirect contact is 48 mW at pipe slop of \({30}^{\circ }\) to 35 mW at pipe slop of \({90}^{\circ }\), more than direct contact. Since the connection of cold surfaces of TEGs happens indirectly through expensive metal plates such as copper and aluminum, therefore, the results of this research can be useful in the economic evaluation and improvement of such devices.

Waste heat recovery seems technically and economically feasible to extract more power from the available system. Thermoelectric-based waste heat recovery is one such technique for direct heat-to-power conversion. Improvement in heat transfer is coherent with the efficiency of such systems. In the current study, the thermal and hydraulic performance of the cold side of the thermoelectric generator is investigated by replacing the conventional parallel channel with the channel having non-parallel walls for heat transfer gains with a 0.1% vol. fraction of MXene nanofluid coolant. Experiments are performed under uniform heat flux with convergent, divergent, and flat ducts for three non-parallel angles 00, 10, and 1.50 within the Reynolds number 700-2100. Moreover, all performance is compared subject to the constraints of the same mass flow rate, pressure drop, and pumping power.The result revealed that heat transfer efficiency rises with an increase in the converging and diverging angle. The divergent channel has a 15.62% and 17.18% greater value than convergent and smooth channels, respectively, at a Reynolds number of 2100. Also, compared to parallel channels, the pressure drop is greater in non-parallel channels. D-3 channel, and under identical pumping power and pressure drop conditions, D-2 performs better with a 0.1% vol. fraction MXene nanofluid coolant at identical mass flow rates. The flow acceleration or deceleration caused by the change in the cross-section of the channel has shown a strong influence on its heat transfer characteristic. The proposed design of the cold channel with MXene nanofluid will cause a profound enhancement in the power output of the thermoelectric system.

The present study investigates the behaviour of thermal streaks on a heated foil which is cooled with turbulent flow in a square duct channel. Real-time infrared thermography is used to visualize and measure the spacing between the thermal streaks. A stainless-steel foil with a thickness of 25 microns is cooled by water. The experiments were performed in a range of Reynolds numbers from 5000 to 20000 and Prandtl numbers from 3 to 7. The mean temperature, root-mean-square of temperature and autocorrelation function have been calculated and used to measure the average thermal streak spacing and power spectra in the spanwise and streamwise directions. The root mean square temperature was 0.3 °C to 0.5 °C which corresponds to roughly 10% of the mean temperature difference between foil and water. The uncertainty in mean temperature difference and root mean square temperature was around 5% and 10%, respectively. The measured thermal streak spacing was 100 wall unit to 180 wall unit under the present experimental range. The uncertainty in measured thermal streak spacing was around 2.5%. The effects of Reynolds number, Prandtl number and heat flux on the thermal streak spacing and also on the statistics of the temperature field have been presented and discussed in this paper. A new correlation has been proposed to predict the dimensionless thermal streak spacing. The error in the prediction is estimated within ± 15 %.

This study investigates flow boiling heat transfer and pressure drop characteristics of R407C (zeotropic mixture) in newly shaped copper-made horizontal enhanced and smooth evaporator tubes of 1000 mm employing a vapour compression refrigeration cycle at ambient pressure and saturation temperatures of 15 – 45 °C. The enhanced tube has 60 trapezoidal microfins with a helix angle of 20° and a height of 0.22 mm leading to a 1.83 times increase in surface area. The effects of mass fluxes of 50–250 kg/m2s, and heat fluxes of 10—80 kW/m2 on the heat transfer coefficient (HTC) and pressure drop, were examined. 1.55, and 1.35 times higher mean HTCs, and pressure drops are recorded for the enhanced tubes with ± 9% and ± 14.5% mean absolute errors respectively with the established correlations including smooth tube findings. The results emphasize the importance of using accurate and validated correlations in refrigeration systems. Local Nusselt number variation with Jakob subcooling number and locations over the evaporator and parametric variation studies are also incorporated to analyse and capture the experimental trends comprehensively.

AbstractDue to the excellent heat transfer performance, spray cooling is widely used for heat removal of high-heat-flux electronic devices. In this study, the influence of spray characteristics on spray cooling heat transfer is tested experimentally, where both water and 4% ethanol-water mixture are used as the working fluid. The Sauter mean diameter and droplet number distributions are measured by the Particle/Droplet Imaging Analysis system, and the fluid volumetric flux distributions are measured by the self-designed Precision Mobile Experiment Bench. The results show that the heat transfer efficiency of the 4% ethanol-water mixture is superior to that of water. Especially, the critical heat flux is about twofold that of water for the Spray A nozzle. For a fixed nozzle, the Sauter mean diameter of the 4% ethanol-water mixture is smaller, while the fluid volumetric flux shows an increase under the same conditions. Small Sauter mean diameter and large fluid volumetric flux can induce more droplets to entrain bubbles. Then the mechanisms of improved spray cooling heat transfer are analyzed based on the spray characteristics measurement results. It is revealed that the Sauter mean diameter and mean fluid volumetric flux exert joint influence on the heat transfer efficiency, and the mean fluid volumetric flux has an obvious effect between different nozzles. Two dimensionless heat flux correlations are proposed for the single-phase and nucleate boiling regimes with mean absolute errors of 15.56% and 14.68%, respectively.HighlightsSauter mean diameter of 4 vol.% ethanol-water is smaller than water, but the fluid volumetric flux is higher than water.Small Sauter mean diameter and large fluid volumetric flux induce more droplets generating entrained bubbles, which can enhance the heat transfer performance.Fluid volumetric flux is predominant in improving heat transfer performance, especially in the nucleate boiling regime.Spray characteristics have an obvious influence on the critical heat flux.Dimensionless heat flux is correlated with characteristic numbers integrating Sauter mean diameter and fluid volumetric flux.

Special flame merging behaviour and complex combustion characteristics can be formed under the coupling effect of tangential flow and heat transfer of multiple pool fires (MPFs) in a shaft. In this study, quadruple pool fires (DPFs) evolutionary experiments in shaft were carried out, and the influence of pooled spacing and side slit width of the shaft on flame merging behaviour were analyzed. The primary influencing mechanism of merged flame evolution was expounded from heat transfer and thermal feedback between fire sources. The shape of conical merged fire whirls that can completely fill the gap between the oil pools with flame is confirmed, and its cone angle is determined by the position of outer edge of oil pools. In the formation of merged fire whirls, the critical conditions are that the distance between fire sources is less than three times the pool diameter and the width of side slits is 1/4 of the side length of the shaft, respectively. Compared with the single pool fire whirls with the same liquid surface area, the thermal flow field of DPFs in shaft is relatively unfavorable to the formation of fire whirls. The formation time of merged fire whirls is nearly five times that of the single pool fire whirls, and the flame height can be reduced by nearly half.The evolution process of the DPFs that can form merged fire whirls exhibits the multi-stage morphological characteristics over time and all of these stages possess different heat transfer mechanisms. Before forming the merged fire whirls, intermittent merging among pool fires is a necessary stage. And the duration of this stage is relatively less affected by the width of slit on the side of shaft. The transverse force of asymmetric tangential flow on the flame is difficult to be completely overcomed by merged fire whirls. Because of the inclination of merged flame, the overall vertical height is unsuitable as a direct characterization parameter of combustion intensity.

The pillow-plate heat exchanger (PPHE) is a kind of heat exchanger constructed by a set of wavy surfaces like a pillow. In this study, the influence of pillow-plate geometrical parameters (including dimensionless channel height, dimensionless plate width, and pillow plates number) and flow specification (including Reynolds and Prandtl numbers) in terms of derived dimensionless parameters on their thermo-hydraulic performance is evaluated through a comprehensive parametric investigation. The fully developed regime in PPHE channel is assumed and fluid flow, heat transfer, and thermodynamics principles are combined in terms of the entropy generation minimization (EGM) approach. Afterwards, a multi-objective optimization method is applied by using the non-dominated sorting genetic algorithm (NSGA-II) to find the optimal design of PPHE. In this way, the maximization of performance evaluation criterion (PEC) against the minimum total entropy generation for PPHE is eventuated. The behavior of PPHE’s important evaluation criteria are illustrated for different Reynolds numbers from 1000 to 6000 by varying Prandtl number and the proposed dimensionless geometry parameters. Also, contrariness between two parts of non-dimensional entropy generation (NDEG), i.e., thermal and frictional, is concluded from the result for Pareto-optimal front. It indicates there is an optimum Re number that minimizes (NDEG)tot at any geometrical parameters. On the other hand, multi-objective optimization results show the conflict between two main objective functions namely PEC and (NDEG)tot that reveals any geometrical change to increase in the PEC of heat exchanger leads rising in total entropy generation and vice versa. The final optimum values of the objective functions are PECopt = 1.3712 and (NDEG)tot,opt = 0.0145 which occurs at Re = 3265.

The process of steam condensation in the presence of non-condensing air inside channels of Plate Heat Exchanger (PHE) is investigated based on experimental data and the results of mathematical modelling. The model consists of a system of ordinary differential equations with nonlinear right parts, solved numerically on a computer. It describes the local process parameters distribution in channel. Each of the PHE tested samples consists of four corrugated plates with three channels between them for the flow of hot and cold streams. The experimental samples have different angles of corrugations to the longitudinal plate axis (60°, 45° and 30°). The applicability of heat-momentum and heat-mass transport analogies for such channels was confirmed, as also the preservation of the Equation for the correction on effect of transverse mass flux in criss-cross flow channels. The specific form of such Equation is proposed. It allows mathematical modelling of steam-air mixture condensation using single-phase correlations for PHE channels. The intensification of overall heat transfer compared to condensation in flat wall channels is observed. It enables the decrease of PHE heat transfer area from 2 up to 4 times compared to a flat wall channel for the same process conditions.

In this study, the effects of Al2O3–water nanofluids on heat-transfer enhancement in the annulus side of a double-tube heat exchanger with a dimpled outer tube was examined. The inline dimples on the outer tube were designed and built to conduct the experiment with the inner tube being the smooth tube. Al2O3 nanoparticles with 0.1%, 0.6%, 1%, and 2% volume concentrations and pure water as a base fluid were obtained. The Reynolds number based on the hydraulic diameter for the annulus region between about 650 and 1750 was considered. The results showed that the annulus Nusselt number of an annulus using the dimpled outer tube increased by 4.7%, 12.0%, 28.8%, and 32% at 0.1%, 0.6%, 1%, and 2% nanoparticle volume concentrations, respectively, compared to pure water at the maximum Reynolds number. The thermal performance factor achieved with the use of Al2O3–water nanofluid in the smooth outer tube tended to be greater than that achieved with the use of Al2O3–water nanofluid in the dimpled outer tube. In addition, new empirical correlations for predicting the annulus Nusselt number for nanofluids in both smooth and dimpled outer tubes are also proposed in this study.