Volumetric Heat Transfer Coefficient Research Articles

Study on volumetric heat and mass transfer coefficient in a spray drying process utilizing pressure swirl nozzle: A numerical and experimental approach

This paper applies a sectional and cumulative approach to estimate the volumetric heat and mass transfer coefficients (hV and kV) to characterize the behaviour of the spray drying process. The investigation utilizes a tailor-made spray dryer equipped with a pressure swirl nozzle with a 0.2 mm orifice diameter, utilizing Mannitol solution as the precursor and air as the carrier gas. A comprehensive numerical analysis is conducted using the Adaptive Mesh Refinement (AMR) technique alongside selective experiments. The study considers precursor mass flow rates of 1 g/s, 1.2 g/s, and 1.5 g/s, along with carrier gas velocities of 0.88 m/s and 1.19 m/s at a temperature of 250 °C. Comparisons between predicted and estimated hV and kV are performed, leading to the proposal of a method for identifying constant rate and falling rate periods. The analyses contribute to delineating the solvent evaporation zone, solid particles heating zone, spray penetration length, and drying length. Additionally, experimental temperature measurements in both axial and radial directions at 12 grid points in the chamber are conducted to validate the numerical results, showing acceptable deviation.

Investigation of mixing and heat transfer characteristics of long-hole SK direct contact heat exchanger

In this study, in order to improve the efficiency of direct contact heat exchanger (DCHE), SK static mixers with different perforation index (PI) were applied. Numerical simulation (volume of fluid model) were used to investigate the performance of the static mixer, which shape was designed as long-holes, under fixed aspect and helical ratios. The feasibility and accuracy of numerical simulation results were verified by self-established direct contact heat transfer (DCHT) experimental platform. Under six different PI values (0 %, 10 %, 18 %, 26 %, 32 %, and 38 %), the heat transfer performance of DCHE was evaluated by applying two heat transfer mediums (THERMINOL62 and R141b). The results indicated that the gaseous working medium outlet temperature, turbulence intensity, turbulence kinetic energy, and volumetric heat transfer coefficient (VHTC) of the system showed an increasing trend as PI increased from 0 % to 32 %, then a decreasing trend can be observed from 32 % to 38 %. The maximum turbulence intensity and turbulence kinetic energy of 17.56 % and 0.015 m2/s2 can be found in PI = 32 %, as well as a maximum enhancement ratio of 49 % in VHTC compared to PI = 0 % can be achieved. In addition, only a small pressure drop increase is observed in PI = 32 % compared to PI = 0 %.

Multi-objective optimization study of airfoil fin printed circuit heat exchanger with tip gap based on machine learning

The printed circuit heat exchanger (PCHE), a compact and highly efficient device, is capable of operating effectively under demanding conditions, which makes it ideal for supercritical CO2 Brayton cycles. In this study, we present a novel airfoil fin PCHE with tip gap, and conduct multi-objective optimization on the tip gap and arrangement of airfoil fins based on the analysis of the supercritical CO2 flow and heat transfer characteristics. We conduct a parameter design using Design of Experiment to examine the impact of the dimensionless design parameters, including the horizontal number (ζh), staggered number (ζs), vertical number (ζv), and gap number (ζg). To forecast the flow and heat transfer performance, we utilize a neural network model called Particle Swarm Optimization-Back Propagation (PSO-BP). We employ the non-dominated sorting genetic algorithm II to obtain the Pareto optimal front by utilizing ζh, ζs, ζv, and ζg as variables for optimization, and the volumetric heat transfer coefficient (hv) and Fanning friction factor (f) as objectives for optimization. The results find that the utilization of tip gap can enhance heat transfer while reducing flow resistance. The PSO-BPNN model exhibits higher prediction accuracy and excellent generalization ability compared with traditional BPNN model. The VIKOR and TOPSIS methods identify compromise schemes with excellent thermal-hydraulic performance. The mechanism of heat transfer enhancement can be elucidated using the principle of field synergy.

Numerical study of particle behaviours and heat transfer in a complex rotary kiln

Rotary kiln is widely used for thermal disposal of solid waste due to its effectiveness and high efficiency in recent years. To further improve the processing efficiency, a newly designed rotary kiln with three-section structure is proposed, and the behaviours of particle motion and heat transfer are investigated. Firstly, a lab-scale rotary kiln is manufactured, and experiments are carried out. Verified by experimental data, a CFD-DEM numerical model is developed to analyze the particle motion and heat transfer characteristics with the effects of inlet flue gas temperature, feeding rate and rotating speed. The results show that the outlet temperature increases linearly with the flue gas temperature, while it is negatively correlated with the feeding rate and rotating speed. In addition, the volumetric heat transfer coefficient in this complex rotary kiln is analyzed, the overall heat transfer coefficient is between 200 and 700 W/(m3 K).

Production of MgAl/layered double hydroxide microspheres and investigation of the volumetric heat transfer coefficient in spray drying

Layered Double Hydroxides (LDH) are synthetic materials nanostructured in two dimensions that present positively charged layers with interspersed anions for charge and structure balancing. Being recognized as a promising material for various applications, a complete exploration of its possible attractive properties and its synthesis process is essential. However, drying, a necessary step in the process, is still little studied. This work aimed to produce MgAl–CO3/LDH microspheres and calculate the volumetric heat coefficient in spray drying, evaluating the drying air inlet temperature and the concentration of the feed paste in the dryer. LDH synthesis was carried out using the coprecipitation method, maintaining a 2:1 Mg/Al ratio. The infrared spectra presented the bands characteristic of the hydrotalcite-type material. Through XRD, it was possible to observe that the variation in drying air temperature and feed paste concentration produced LDHs with structural differences. The results obtained for the basal spacing ranged from 7.685 to 7.705 Å. Scanning electron microscopy images confirm the production of LDH microspheres, showing variation in the size of the agglomerates with changes in the feed paste concentration. The volumetric heat transfer coefficient values ranged from 4.31 to 5.36 W m−3 K−1, with only the air inlet temperature significantly influencing the process under the conditions studied.

Experimental and analytical study on continuous frozen/melting processes of latent thermal energy storage driven by bubble flow

It is emphasized direct contact method that introduced to the frozen/melting processes has a high value of development due to the role for mitigating thermal stratification, reducing phase separation and enhancing heat transfer efficiency. However, the absence of fixed solid heat transfer walls can lead to discontinuous switch and difficult-to-control problem during frozen/melting processes. In this paper, the latent thermal energy storage device driven by bubble flow is experimentally investigated. With the usage of low melting point paraffin and the pattern of free expansion in a cabinet, continuous switch between the frozen and the melting process can be realized. Phase change processes, temperatures, heat transfer efficiency, volume change and pressure drops are measured and analyzed. A dynamic analytical model to describe the processes is also developed based on the ε - NTU method. The results show that the heat transfer efficiency can reach to 0.8 at least in all the experimental cases. Moreover, this study records the pressure difference of the gas flow, predicts the required input mechanical work and analyses the volumetric heat transfer coefficient, which help to reveal feasibility of the device.

Influence mechanism of the three-zone synergistic efficiency-enhancing pattern on the 3D flow field and thermal resistance characteristics for wet cooling towers

For a super-large natural draft wet cooling tower (S-NDWCT) equipped in a 1000 MW unit, a 3D numerical model of the three-zone synergistic efficiency-enhancing pattern (TSEP) was established and validated. This study focused on the influence mechanism of the three-zone parameters on the 3D flow field and thermal resistance characteristics, including the installation height h, installation angle θ, total coverage area S, length L and number n of the split-flow plate, water distribution partition radius R1, the proportion of inner zone water amount P, and fillings partition radius R2. The results showed that the TSEP reconfigures and comprehensively optimizes the airflow field, and presents the more uniform air-water ratio and water temperature fields. The volumetric heat transfer coefficient hv increases and then reduces as h, θ, S, n, R1, P and R2 increase, and increases continuously with increasing L in the range studied. Additionally, the ventilation G increases and then reduces with the increase of h, θ, R1 and P, increases with rising S, n and R2, and reduces with L increases. Finally, the relatively optimized values of the three-zone parameters are obtained. In this case, compared with a conventional tower, the hv and G increase by 11.3% and 10.6%, respectively.

Research Progress on Direct Contact Evaporation Heat Transfer between Two Immiscible Working Fluids

AbstractThe direct contact evaporation heat transfer of two immiscible fluids can realize low‐temperature differential heat transfer, which is an important method to improve the heat transfer coefficient. This article focuses on the dispersed phase of organic working fluids and the continuous phase of immiscible inorganic fluids and summarizes the research on direct contact evaporation heat transfer in the past 15 years. The main research methods include theoretical analysis, numerical models, and experimental research. The evaporation process of a single droplet mainly involves bubble droplet growth, droplet configuration and discriminant, droplet shape change, rising speed and path, etc. In practice, dispersed phases mostly exist in populations, and the research mainly focuses on collisions, coalescence, and rupture between droplets and bubbles, as well as on the number and size distribution. The volumetric heat transfer coefficient is used to reflect the heat transfer capacity of the heat exchanger. The factors affecting evaporation heat transfer performance are complex, and increasing the uniform mixing of bubbles is an important method to improve the heat transfer coefficient. In the future, direct contact evaporation heat transfer is expected to be promoted to multiple fields.

New structure-performance relationships for surface-based lattice heat sinks

Heat sinks have manifold applications, from micro-electronics to nuclear fusion reactors. Their performance expectations will continue to increase in line with the power consumption and miniaturisation of technology. Additive manufacturing enables the creation of novel, compact heat sinks with greater surface-to-volume ratios and geometrical complexities than standard pin/fin arrays and pipes. Despite this, there has been little research into the use of high surface area lattice structures as heat sinks. Here, the hydraulic and thermal performance of five surface-based lattice structures were examined numerically. Computational fluid dynamics was used to create useful predictive models for pressure drop and volumetric heat transfer coefficients over a range of flow rates and volume fractions, which can henceforth be used by heat transfer engineers. The thermal performance of surface-based lattices was found to be heavily dependent on internal geometry, with structures capable of distributing thermal energy across the entire fluid volume having greater volumetric heat transfer coefficients than those with only localised areas of high heat transfer and low levels of fluid mixing.

Numerical and experimental investigation of additively manufactured shell-lattice copper heat exchanger

Triply periodic minimal surface (TPMS) shell lattices have superior heat transfer properties due to their staggered channels. Based on these structures, a series of TPMS heat exchangers (HEs) are fabricated and investigated. However, the existing research is mainly limited to the performance investigation of HEs based on low-thermal-conductivity materials or the simulation study of the representative parts. In this study, we successfully fabricated a pure copper TPMS HE and performed a comprehensive numerical analysis. Three HEs, including Copper TPMS, SS316L TPMS, and SS316L plate were studied and compared to reveal the mechanism of their performance differences. Numerical analysis showed that TPMS HEs owned more uniform temperature and velocity fields than plate HE. The calculated heat-exchange effectiveness of the copper TPMS HE is about 58%–96% higher than that of SS316L plate HE, while the measured value of that of SS316L TPMS HE is about 38%–48% more than that of SS316L plate HE, respectively. The convective heat-transfer coefficient of copper TPMS HE is approximately 2.16–2.69 times that of SS316L plate HE and 1.17–1.33 times that of SS316L TPMS HE. The volume goodness factors of copper TPMS and SS316L TPMS HEs are about 1.49–1.87 and 1.2–1.41 times that of SS316L plate HE, respectively. The volumetric heat-transfer coefficient of copper TPMS HE is much better than that of metal plate HE and approximately 18–25 times that of the polymer TPMS HEs. This study indicates that the pure copper TPMS HE is a good candidate for next-generation HEs.

Experimental investigation on volumetric heat transfer coefficient during intermittent spray drying of mannitol solution

The paper presents a detailed experimental investigation on volumetric heat transfer during intermittent spray drying of a 20% w/w of mannitol solution in a custom-made spray dryer. Unlike the conventional intermittent drying process, which achieves intermittency by pulsating the properties of the carrier gas medium, the present works achieve intermittency by pulsating the nozzle spray. The estimated volumetric heat transfer coefficient varies between 0.8 and 2 kW/m3K, and the volumetric mass transfer coefficient is between 0.6 and 2.5 s−1. The correlation for heat and mass transfer coefficients is evolved that relates the heat and mass transfer coefficient with the flow properties and thermophysical properties of both the carrier gas and the precursor fluid in terms of Ohnesorge number and Nusselt number. The investigation uses response surface analysis to analyze the effect of Reynolds number of air and Ohnesorge number of precursors on the volumetric heat transfer coefficient. The results show that a decrease in Reynolds number for a wide range and Ohnesorge number causes the volumetric heat transfer coefficient to increase.

Characterization of bubble population morphology in direct contact heat transfer process under different flow regimes and its influence on heat transfer performance

In this study, the effect of bubble population morphological characteristics and heat transfer performance in the direct contact boiling heat transfer process under two different flow modes are comparatively investigated. Based on the visualization method and homogeneity theory, the relationships between the bubble population morphological characteristics, distribution homogeneity, specific interface area, and volumetric heat transfer coefficient were investigated. The obtained results show that the bubble morphology tends to flatten with the increase of the dispersed phase flow rate in both flow modes. The more uniform the distribution of the bubble population, the better the specific interface area, and the trend is more obvious in the two-phase reverse flow mode. In addition, it was found that the specific interface area was positively correlated with the volumetric heat transfer coefficient. The synergistic relationship between the uniformity of bubble population distribution and the improvement of heat transfer is explained from the perspective of a specific interface area. It is concluded that different flow patterns affect the morphological characteristics of the bubble population and its distribution, which in turn affects the specific interface area of the two-phase contact heat transfer and ultimately the heat transfer performance. The morphology of the bubble population and its uneven distribution has a strong correlation with the flow pattern. This thesis provides a new idea to study the influence of bubble cluster morphology characteristics and heat transfer performance in direct contact boiling heat transfer process under different flow patterns and provides a new way for heat exchanger design.

A Critical Evaluation of Recent Studies on Packed-Bed Bioreactors for Solid-State Fermentation

Packed-bed bioreactors are often used for aerobic solid-state fermentation, since the forced aeration supplies O2 and removes metabolic heat from the bed. Motivated by the potential for applications in biorefineries, we review studies conducted on packed-bed bioreactors over the last decade, evaluating the insights these studies provide into how large-scale packed beds should be designed and operated. Many studies have used low superficial air velocities and suffer from preferential airflow, such that parts of the bed are not properly aerated. Moreover, some studies have proposed ineffective strategies, such as reversing the direction of the airflow or introducing air through perforated pipes within the bed. Additionally, many studies have used narrow water-jacketed packed-bed bioreactors, but these bioreactors do not reflect heat removal in wide large-scale packed beds, in which heat removal through the side walls makes a minor contribution. Finally, we conclude that, although some attention has been given to characterizing the porosities, water sorption isotherms and volumetric heat and mass transfer coefficients of substrate beds, this work needs to be extended to cover a wider range of solid substrates, and work needs to be done to characterize how these bed properties change due to microbial growth.

Heat and mass transfer characteristics during spray drying of Na2Fe0.6Mn0.4PO4F/C cathode material for Na-ion batteries

The paper presents a detailed experimental investigation on heat and mass transfer characteristics during the synthesis of Na2Fe0.6Mn0.4PO4F/C using intermittent spray drying. The drying process is carried out in a custom-made spray dryer. The novelty of the work is the study of the effect of intermittency on the spray drying process heat and mass transfer characteristics using a volumetric approach. The nozzle orifice diameter, spray duration, and spray pressure are operating parameters of the process that are varied between 0.2 and 0.4 mm at 10 and 40 sand 4 and 7 bar. An increase in nozzle orifice diameter and pumping pressure improves the heat transfer significantly; however, an increase in spray time diminishes the heat transfer. Under considered operating conditions, the volumetric heat transfer coefficient varied between 1.5 and 3.5 kW/m3K. The volumetric mass transfer coefficient ranges between 2.5 and 5.0 s−1. Based on the study, correlations for heat and mass transfer coefficients have been developed that relate the thermophysical properties of both the carrier gas and the precursor fluid in terms of Ohnesorge number and Reynolds number.

Pre-Drying of Chlorine-Organic-Contaminated Soil in a Rotary Dryer for Energy Efficient Thermal Remediation.

In response to the current problem of the high energy consumption of direct thermal desorption systems when treating soils with a high moisture content, we propose using the waste heat of the system to pre-dry soil to reduce its moisture. Taking chlorine-organic-contaminated soil as an object, an experimental study on the drying and pollutant desorption characteristics of soil in an indirect rotary dryer was carried out. The results show that the non-isothermal drying process was divided into warm-up and falling rate periods, and no constant period was observed. The higher the rotation speed, the lower the soil outlet temperature and the higher the drying tail gas temperature. Soil outlet and dry tail gas temperatures were lower for soils with a higher moisture content. Benzene and cis-1,2-dichloroethylene are easily desorbed. Therefore, the disposal of dry tail gas should be determined according to the type and concentration of soil pollutants present. The volumetric heat transfer coefficient was found to be 85-100 W m-3 °C-1, which provides a key parameter for the size design of a rotary dryer.

Determining Hydraulic Resistance and Volumetric Heat and Mass Transfer Coefficients during Cooling of Circulating Water in a Multistage Vortex Chamber

Determining Hydraulic Resistance and Volumetric Heat and Mass Transfer Coefficients during Cooling of Circulating Water in a Multistage Vortex Chamber

Enhanced of fluids mixing and heat transfer in a novel SSK static mixer Organic Rankine Cycle direct contact heat exchanger

In this study, new gas-liquid-liquid three phase direct contact heat exchangers based on SSK mixing elements and annular ribs are proposed to improve the thermal efficiency of the Organic Rankine Cycle. Three types of heat exchangers, including hollow tube heat exchanger, SSK heat exchanger and SSK annular rib combined heat exchanger, are established to compare their heat transfer performance. The heat transfer performance, turbulence intensity, turbulence kinetic energy and pressure drop of three heat exchangers are numerically investigated. The results show that the volumetric heat transfer coefficient, outlet temperature of gaseous working medium, turbulence intensity and turbulent kinetic energy of the combined heat exchanger are significantly higher than those of the SSK heat exchanger and the hollow tube heat exchanger. In addition, the pressure drops of the SSK heat exchanger and the combined heat exchanger are basically the same, which are slightly larger than the pressure drop of the empty tube heat exchanger. In summary, the combined heat exchanger has improved heat transfer performance without causing excessive pressure drop.

Numerical and experimental investigation of surface-stabilized combustion in a gas-fired condensing boiler

Gas-fired condensing boilers are widely used in domestic heating. However, emission of nitrogen oxides (NOx) remains a concern due to their adverse environmental effects. Employing lean premixed surface-stabilized combustion, ultra-low NOx generation is achievable.We studied NOx formation in realistic, axi-symmetric condensing boiler setups featuring lean combustion of methane stabilized by a porous cylindrical burner using a dedicated numerical model. To accurately represent the physical conditions, our model comprises most notably laminar combustion using a GRI-3.0 kinetic mechanism, a volumetric heat transfer coefficient for heat exchange inside the porous solid, conjugate heat transfer at the gas-solid interface and radiative heat exchange between burner surface and combustion chamber wall. A comprehensive assessment included validation of individual model aspects against literature data, a mesh study and validation with experimental measurements from a full-scale condensing boiler setup. Our findings suggest that the effective diffusion due to dispersive flow in porous media is critical for accurately capturing the reaction zone, thus preventing overprediction of flame temperature and NO concentration.The dependencies of NOx generation and heat release parameters on varying parametric and geometric configurations of the setup were assessed in a comprehensive numerical parameter study. We found that NO emission increases in a near-linear way with firing rate, while radiation efficiency decreases. At constant input power, NO emission rises with increasing equivalence ratio alongside radiation efficiency and peak flame temperature. By optimizing inflow uniformity, peak temperatures can be reduced by 9%, leading to significantly lower NOx generation without affecting the heat flow rate.

A Multi-Objective Comprehensive Evaluation of Heat Transfer Performance of a Direct-Contact Heat Exchanger

Abstract Direct-contact heat exchangers have great potential for waste-heat utilization. This study established a multi-objective evaluation model of the volumetric heat-transfer coefficient, entransy dissipation, and uniformity of the temperature difference field of the direct-contact heat exchanger. The optimal operating parameters for the heat exchanger were obtained by experimentally investigating the relationship between the operating parameters and the volumetric heat-transfer coefficient, entransy dissipation, and uniformity of the temperature difference field. The results indicated that the smaller the entransy, the greater the effectiveness of the heat exchanger in the direct-contact heat exchanger. The results also indicated that the volumetric heat-transfer coefficient increased with the increase in the flowrate ratio and initial temperature difference. The study found that entransy demonstrated the smallest dissipation and the largest effectiveness of the heat exchanger when the initial temperature difference was 120 °C and the flow ratio was 6:1. This study has guiding significance for the optimal design of direct-contact heat exchangers.

Visualization of direct contact heat transfer process driven by continuous low-temperature heat source and its performance characterization

ABSTRACT In this study, the uniformity factor of the temperature difference field was introduced into the direct contact heat exchanger by establishing a trace element layer model for the heat exchanger. Furthermore, the uniformities of the temperature and temperature difference field of the direct contact heat exchanger operated using a continuous heat source were analyzed. The heat exchange height was proposed,and its effects on the volumetric heat transfer coefficient and heat-exchanger efficiency were analyzed. The experimental results were consistent with the uniformity principle of the temperature difference field.