Volumetric Heat Transfer Coefficient Research Articles

The Mean Temperature Difference Method for Micro Heat Exchanger Analysis Considering Property Variation

ABSTRACTThe influence of reducing dimensions to microscale on the performance parameters of parallel flow tubular micro heat exchanger is numerically analyzed considering the thermophysical property variation effect. Also, the improvement in convective heat transfer coefficient, at a given temperature, with the decrease in dimensions to microscale is investigated. In the micro heat exchanger, conventional method fails to give accurate performance analysis due to significant property variation. So, a new temperature difference, i.e. mean temperature difference correlation considering the property variation effect is derived. Operating conditions and geometric parameters are varied, keeping the inlet temperature of the both fluids and inlet velocity of the cold fluid fixed. The following performance parameters are evaluated: effectiveness, volumetric heat transfer coefficient, mean temperature difference, and log mean temperature difference. The reliability of the new mean temperature difference method is checked by calculating the percentage deviation between mean temperature difference and log mean temperature difference value for macroscale to microscale heat transfer. The results show that percentage deviation is maximum at microscale.

Volumetric Convective Heat Transfer Coefficient Model for Metal Foams

ABSTRACTThis study investigates the heat transfer in metal foams assuming that all the foam cells are cubic. An improved model for the volumetric convective heat transfer coefficient of metal foam was obtained by considering the convective heat transfer and the thermal conduction in the connected foam ligaments. Numerical simulations of a metal foam-filled channel were performed using the local thermal non-equilibrium model to evaluate the model. The theoretical and experimental data agree better with the present model than with other models. The velocity gradient near the metal foam-filled channel wall is greater than in an empty channel, with the wall boundary layer becoming thinner. The temperature differences between the fluid and solid phases are predicted by the model with the pore density affecting the volumetric convective heat transfer coefficient more than the porosity. The volumetric convective heat transfer coefficient significantly increases as the pore density increases. However, the relationship between the volumetric convective heat transfer coefficient and the porosity is not linear. As the porosity increases, the volumetric convective heat transfer coefficient first increases and then decreases. The effect of the solid thermal conductivity on the volumetric convective heat transfer coefficient has an upper limit with a critical value of the solid thermal conductivity for the present conditions.

Thermochemical storage analysis of the dry reforming of methane in foam solar reactor

The high-temperature heat transfer and thermochemical storage performances of dry reforming of methane in a foam reactor subjected to highly concentrated solar radiation is numerically investigated. Two new correlations for the volumetric heat transfer coefficient and the pressure-drop within foam structure are proposed through several experimental tests. Temperature and species distributions inside the reactor as well as the overall methane conversion and thermochemical energy storage efficiency under various operating conditions are predicted. The results indicate that the increase in inlet velocity and CH4/CO2 ratio produces a reduction in methane conversion, while increasing the solid-phase thermal conductivity promotes the conversion. The energy storage efficiency exhibits non-monotonous change with inlet velocity, feed ratio, and foam structural parameters. Compared to other foam structural parameter combinations, maximum conversion and efficiency are observed at porosity of 0.9 and pore diameter of 1.5 mm. Besides, the solid-phase thermal conductivity and foam structural parameters exert negligible effect on the H2/CO ratio in the reforming process.

Volumetric Heat Transfer Coefficient in Fluidized‐Bed Dryers

AbstractIn literature, there are several Nu=f(Re) equations to specify the heat transfer coefficient between solids and the drying gas, but these equations differ significantly because of the inaccuracy of determining the contact surface between the two phases. A pilot‐plant fluidized‐bed dryer is developed to study the heat and mass transfer phenomena during the drying process. A volumetric heat transfer coefficient is applied for modeling fluidized‐bed dryers. A modified Nusselt number is defined to compare the experimental results and those of the literature. The modified Nu'=f(Re) equation exhibits a proper correlation between the results of the experiments in the literature and those of our experiments.

Relation of the volumetric mass and heat transfer coefficients in a cooling tower

Relation of the volumetric mass and heat transfer coefficients in a cooling tower

Experimental investigation of the effect of different nanofluids on the thermal performance of a wet cooling tower using a new method for equalization of ambient conditions

In this study, first a new method has been presented for equalization of ambient conditions in different experiments on cooling towers. Next, using this method, the effect of four nanofluids including Zinc Oxide/Water, Silica/Water, Alumina/Water, and Graphene/Water has been investigated experimentally on the thermal performance of a wet cooling tower with a cross flow. The nanofluids have been studied at concentrations of 0.02, 0.05, and 0.08 wt% and the parameters of cooling range, effectiveness of the cooling tower, evaporation rate, the cooling tower characteristic, and volumetric heat transfer coefficient have been calculated and compared with each other. The different behavior of these nanofluids on the thermal performance of the cooling tower has also been indicated. For example, with the use of Graphene/Water at 0.02 wt%, the effectiveness and cooling tower characteristic have increased by 8.3% and 36%, respectively in comparison with pure water. On the other hand, with the use of Alumina/Water nanofluid across all the concentrations, the thermal performance of the cooling tower has weakened in comparison with pure water, and for example at 0.08 wt% of this nanofluid, the cooling tower characteristic has diminished by 14.7% in relation with pure water. Furthermore, by comparing the investigated nanofluids, it was found that Graphene/Water has resulted the best thermal performance in the cooling tower across all the concentrations, when compared with other investigated nanofluids. Moreover, the best thermal performance of the cooling tower by Graphene/Water has been obtained at 0.02 wt%, at which the volumetric heat transfer coefficient has increased by 36.2%, in comparison with pure water.

Experimental Investigations of a Pilot Scale Coal Impinging Stream Dryer

ABSTRACT A method for drying hard coals using an impinging stream dryer has been proposed in the article. A series of screening tests were conducted, seeking optimum process parameters of an innovative coal impinging stream dryer using flue gas as the drying medium. The article addresses measurement results obtained for the flue gas and feedstock flow rate, feedstock sort, and initial moisture content on the specific energy consumption, volumetric water evaporation rate, and moisture reduction. The maximum volumetric water evaporation rate was found to be around 230 kgwater/m3·h and the maximum volumetric heat transfer coefficient came to ca. 2.5 kW/m3·°C. In terms of specific energy consumption, the lowest specific energy consumption was found to be around 1.79 kWh/kgwater. It was established that the relation between specific energy consumption and a product of the flue gas flow rate and the initial water (in the feedstock) flow rate is a power function. The best performance of the impinging stream dryer was achieved for high-moisture coal and high flue gas flow rates. The impinging stream dryer proved to be suitable for surface moisture removal from particulate materials that contain very high moisture. The initial tests conducted at the pilot plant successfully demonstrated reliable operation and promising results.

Transient Heat Transfer Study of Direct Contact Condensation of Steam in Spray Cooling Water

We conducted a transient experimental investigation of steam–water direct contact condensation in the absence of non-condensible gas in a laboratory-scale column with the inner diameter of 325 mm and the height of 1045 mm. We applied a new analysis method for the steam state equation to analyze the molar quantity change in steam over the course of the experiment and determined the transient steam variation. We also investigated the influence of flow rates and temperatures of cooling water on the efficiency of steam condensation. Our experimental results show that appropriate increasing of the cooling water flow rate can significantly accelerate the steam condensation. We achieved a rapid increase in the total volumetric heat transfer coefficient by increasing the flow rate of cooling water, which indicated a higher thermal convection between the steam and the cooling water with higher flow rates. We found that the temperature of cooling water did not play an important role on steam condensation. This method was confirmed to be effective for rapid recovering of steam.

Anisotropy effects on convective heat transfer and pressure drop in Kelvin’s open-cell foams

Open-cell foams have the potential to offer heat transfer enhancement in many applications, such as heat exchangers, solar air receivers, porous burners, thanks to their high heat transfer surface to volume ratio and to tortuosity, that promotes the internal flow mixing. Their microstructural transport features are affected by foam anisotropy, and could play an important role not only because manufacturing processes can stretch the cells along a preferential direction, but also because modern techniques allow for the customization of the foam cell shape. However, structural anisotropy in open cell foams has been scarcely investigated. In this paper, anisotropy effects on convective heat transfer and pressure drop in Kelvin’s open-cell foams are analyzed. Since Kelvin’s model is geometrically regular, anisotropy has been investigated by stretching the foam along three orthogonal directions, at equal cell volume. Governing mass, momentum and energy equations have been solved using a finite element method. Results are presented for different fluid inlet velocities and cell sizes along three orthogonal stretching directions. They show that anisotropy affects the velocity and temperature fields, and, consequently, the permeability, inertial factor and volumetric heat transfer coefficients.

Rotor‐Stator Spinning Disc Reactor: Characterization of the Single‐Phase Stator‐Side Heat Transfer

AbstractThe single‐phase fluid‐stator heat transfer in a rotor‐stator spinning disc reactor in dependence on rotational Reynolds number, dimensionless throughput, Prandtl number, and aspect ratio is examined. For the selected ranges of these parameters, an increase in the stator‐side Nusselt number with increasing Reynolds number, Prandtl number, and a higher throughput is found. Laminar and turbulent flow regions are observed, which coincide with a throughput‐ and a rotation‐governed heat transfer regime, respectively. A Nusselt correlation to predict the experimental data in the turbulent flow regime within 20 % accuracy was established. A distinct increase in the overall volumetric heat transfer coefficient for a rise in Reynolds number was observed, being considerably higher compared to conventional tube reactors and twice as large in contrast with a similar rotor‐stator setup.

Heat Transfer During High Temperature Gas Flow Through Metal Foam Heat Exchangers

An experimental study of heat transfer in metal foam heat exchangers fabricated from 10 and 40 pores per inch (PPI) was conducted. Heat exchangers were made by either brazing Inconel sheets to foam or plasma spraying Inconel skins on the foam. A burner test rig was built to produce high temperature combustion gases at either 550 °C or 750 °C that were passed over the exposed surface of heat exchangers that were cooled by passing air through them at rates of up to 200 SLPM. Both pressure drop and temperature rise of the air were measured. Friction factors and volumetric heat transfer coefficients were calculated for air velocities varying from 0.1 to 5 m/s and dimensionless correlations to predict these derived. The heat exchangers with 40 PPI foam were measured to have higher heat transfer rates and larger pressure drop than those with 10 PPI foam. Thermal sprayed heat exchangers were found to perform better than those that were brazed since they had lower thermal contact resistance between the external shell and foam struts. An analytical model was developed assuming local thermal nonequilibrium (LTNE) and predictions from model were found to be in good agreement with experimental results.

Reticulated porous volumetric solar receiver designs guided by normal absorptance and hemispherical volumetric emittance investigations

Here, we designed volumetric receivers from the angle of their normal absorptance and hemispherical volumetric emittance. We showed that the optimal receiver with the reticulated porous structure to date should have 95% porosity, 0.4mm average pore diameter and 8mm length. This receiver has both high normal absorptance and low hemispherical volumetric emittance because these geometric parameters are designed to achieve the minimum optical thickness that is sufficient to achieve the largest possible normal absorptance. In addition, its high porosity and small pore size achievable using replication technique contribute to high specific surface area and resultant high volumetric convective heat transfer coefficient. Our results demonstrated that the thermal efficiency can be potentially further enhanced by integrating textures comparable to the wavelength within the millimeter-scale pores to tune radiation as well as convection properties.

Transient analysis of the steam‐water direct contact condensation in the packed column

AbstractThe transient heat transfer behaviour of direct contact condensation of steam in spray cooling water in a packed column was investigated experimentally and theoretically in this work for the first time. A new analysis method of the state equation was applied to analyze the molar quantity variation of steam in the course of the experiment. The results showed that increasing the cooling water flow rate properly could significantly accelerate the steam condensation rate and this was more obvious at the beginning of the operation. The higher the flow rate of the cooling water was, the shorter a time it took for the steam to be completely condensed. When the cooling water flow rate was 840 L/h, the steam was mainly condensed within 13 s, which indicated that it would rapidly handle steam by regulating cooling water flow rate. However, the effect of changing cooling water temperature on steam condensation rate is slight with the cooling water temperature inceasing from 23 °C to 42 °C. The volumetric heat transfer coefficient in this experiment is in the range of 1.47 kW · m−3 · K−1 to 10.93 kW · m−3 · K−1 with the cooling water flow rate inceasing from 120 L/h to 840 L/h, and the maximum uncertainty in the results is 2.2 %.

A Markov Chain Monte Carlo-Metropolis Hastings Approach for the Simultaneous Estimation of Heat Generation and Heat Transfer Coefficient from a Teflon Cylinder

ABSTRACTThis paper reports the use of Markov Chain Monte Carlo (MCMC) and Metropolis Hastings (MH) approach, to solve an inverse heat transfer problem. Three-dimensional, steady state, conjugate heat transfer from a Teflon cylinder of dimensions 100 mm diameter and 100 mm length with uniform volumetric internal heat generation is considered. The goal is to estimate volumetric heat generation and heat transfer coefficient, given the temperature data at certain fixed location on the surface of the cylinder. The internal volumetric heat generation is specified as input and the temperature and heat transfer coefficient values are obtained by a numerical solution to the governing equation. The temperature values also depend on heat transfer coefficient which is obtained by solving Navier–Stokes equation to obtain flow information. In order to reduce the computational cost, a neural network is trained from the computational fluid dynamics simulations. This is posed as an inverse problem wherein volumetric heat generation and heat transfer coefficient are unknown but the temperature data is known by conducting experiments. The novelty of the paper is the simultaneous determination of volumetric heat generation and heat transfer coefficient for the experimentally measured steady-state temperatures from a Teflon cylinder using MCMC-MH as an inverse model in a Bayesian framework and finally, the estimates are reported in terms of mean, maximum a posteriori, and the standard deviation which is the uncertainty associated with the estimated parameters.

Determination of entry length of a fluidized bed dryer using volumetric heat transfer coefficient

Fluidized bed dryers are widely used in several fields of industry. Sufficiently accurate thermal models provide an opportunity to increase the effectiveness of dryers. The required size of a fluidized bed dryer can be defined with the application of mathematical model. This work is aimed at developing mathematical model to investigate the influence of operating parameters in a fluidized bed dryer using volumetric heat transfer coefficient. After the defining the input parameters of the differential equations, the required entry length of the dryer which effective heat and mass transfer between gas and particles takes place can be estimated. The correct estimation of the entry length is useful in optimal design of a fluidized bed dryer. Using the model the impact of the drying parameters can be determined to the required length.

Catalytic combustion of residual methane on alumina monoliths and open cell foams coated with Pd/Co3O4

The reactivity of a catalytic layer consisting of 3wt.% of Pd/Co3O4 over an alumina monolith (100cpsi, diameter 0.9cm, length 3cm) and two alumina open cell foams (30 and 45ppi, diameter 0.9cm, length 3cm) was investigated in the oxidation of residual methane in lean conditions. The ceramic structures were coated via solution combustion synthesis (Co3O4) and wetness impregnation (Pd). The catalytic reactivity of the catalysts coated on the structured supports was assessed in a gas mixture containing 0.5 or 1vol.% of methane at different weight hourly space velocities (WHSV, 30 and 60NLh−1 gcat−1). Moreover, the prepared structured catalysts were characterized by measuring the pressure drop and determining the volumetric heat transfer coefficients. Both the WHSV and the characteristics of the support influenced the catalytic activity. In general, open cell foams coated with Pd/Co3O4 show superior catalytic activity than the coated monolith.

The effect of packing on direct contact evaporation in spray column

AbstractThe direct contact evaporation in countercurrent spray column has been experimentally investigated. Two kinds of random packing were added into the column independently to study their influence on heat transfer process. Three different continuous phase flow rates (150, 200, 250 L/h) and two different dispersed flow rates (6.565, 13.13 L/h) were used in the experiment. The inlet continuous phase temperature ranged from 42 to 57 °C, and the inlet dispersed phase temperature was near its boiling point. The effect of variable factors on the optimal column height, the volumetric heat transfer coefficient, the pressure drop, and the thickness of foam layer were studied. The results showed that packing had positive effects on heat transfer, such as decreasing the optimal column height and the thickness of foam layer, increasing heat transfer coefficient. But extra pressure was also obvious. Dixon rings performed better than Raschig rings in the experiments due to their finer structure.

Measuring the Overall Volumetric Heat Transfer Coefficient in a Vapor-Liquid–Liquid Three-Phase Direct Contact Heat Exchanger

ABSTRACTAn experimental investigation of the volumetric heat transfer coefficient in a three-phase direct contact condenser was carried out. A 75-cm-long cylindrical Perspex column with a 4 cm diameter was used. Only 48 cm of the column was utilised as the active direct contact condensation height. Pentane vapor at three different initial temperatures (40°C, 43.5°C, and 47.5°C), with differing mass flow rates, and tap water at a constant initial temperature (19°C) with five different mass flow rates were employed as the dispersed phase and the continuous phases, respectively. The results showed that the volumetric heat transfer coefficient increased with increasing mass flow rate ratio (variable dispersed phase mass flow rate per constant continuous phase mass flow rate), the continuous phase mass flow rate and holdup ratio. An optimal value of the continuous phase mass flow rate is shown for an individual dispersed phase mass flow rates. This value increases with increasing vapor (dispersed) phase mass flow rate. Furthermore, it was observed that the initial driving temperature difference had no effect on the volumetric heat transfer coefficient. While, the temperature gained by the continuous phase has a considerable effect.

The effect of Dixon rings on direct contact heat transfer performance: Comparison of counter and co-current evaporation

In the experiment, direct contact evaporation in both co-current and counter-current flow were compared with pentane as dispersed phase and water as continuous phase. Dixon rings were also added into the column to enhance heat transfer in those two different contact ways. Axial temperature distribution, optimal column height and volumetric heat transfer coefficient are important parameters to evaluate heat transfer performance. The influences coming from pentane flow rate, water flow rate and inlet water temperature have also been carefully studied. Compared with co-current flow, the optimal column height decreased 15–25% and but heat transfer coefficient increased in the same scale no matter with packing or not in the counter-current condition. The optimal column height was halved and heat transfer coefficient doubled with packing when other factors were the same. Both optimal height and heat transfer coefficient would decrease with the increasing inlet water temperature. Packing in the column was an effective approach to enhance heat transfer performance especially under the condition of low water temperature as well as high water flow rate.

Scale-resolved CFD modelling of single-phase hydrodynamics and conjugate heat transfer in solid sponges

A scale-resolving CFD modelling approach allowing an in-depth analysis of hydrodynamics and heat transfer in irregular, highly porous structures is presented. It has been developed for experimentally well-characterized solid sponges, also often referred to as open-celled foams, but can be applied to similar structures as well. Due to computational cost limitations, it is necessary to restrict the scale-resolved CFD calculations to adequately sized and shaped representative elementary volumes (REVs) of the porous structure only. A special multi-zone approach has been developed to still allow the specification of realistic hydrodynamic and thermal boundary conditions. It also considers the influence of the REV’s real surrounding if inserted for example into a larger pipe or reactor. It relies on the coupling of the scale-resolved REV calculations with so-called porous ‘embedding zones’ which are being modelled as homogeneous porous media having equivalent effective properties. Two different heat transfer models are presented which allow both the separate analysis of convective fluid heat transfer only and the coupled analysis of conjugate fluid-solid heat transfer. Systematic parameter variations (superficial velocity and solid heat conductivity) have been carried out to work out the fluid and solid heat transfer contributions. Finally, results derived for the integral quantities pressure drop Δp and volumetric heat transfer coefficient αV,F-S are compared to the validation data available from literature and show good agreement.