Forced Convection Environments Research Articles

Evaluation of a helical coil heat exchanger in a forced convection environment

Evaluation of a helical coil heat exchanger in a forced convection environment

Disruption and microexplosion mechanisms of burning alcohol droplets with the addition of nanoparticles

Microexplosions of burning liquid fuels have been actively investigated and considered due to the advantage for substantially increasing the overall burning rate. This study concerns possible control of microexplosion occurrence by adding nanoparticles in alcohol fuel without effects of surfactant and ignitable particles as usually seen in the literature. In addition, the essential characteristics and differences between various particles on combustion are discussed. Specifically, we have added three types of ceramic nanoparticles into butanol and isopropanol droplets under different convection environments, i.e., forced convection and natural convection in normal gravity, and free convection in microgravity. A suspended droplet method was performed under convection-free and natural convective conditions while a free-falling droplet method provided a forced convection environment. It was observed that the combustion characteristics of droplets changed from extinction to burnout or from burnout to microexplosion by using different base fuels, by increasing the intensity of flow convection, or by using different kinds of nanoparticles. During combustion, droplets would deform due to the presence of gelation in the outer layer when Al2O3 or SiO2 nanoparticles were added, but not for TiO2. This yielded one of the key factors leading to microexplosions, in addition to the amount of heat absorption during the burning. For the latter, specifically, microexplosions happened when butanol/SiO2 droplets were tested, but not for butanol/Al2O3 droplets. By comparing the infrared absorptivity of Al2O3 and SiO2, distinct amount of absorbed infrared radiation was demonstrated. Such a difference in heat absorption was also key to the creation of microexplosion. As a consequence, microexplosions could be generated when nanoparticles were added into the fuel and formed gelation on the droplet surface, which trapped bubbles in the droplet, while sufficient heat was absorbed by the particles during the burning. The resulted consumption rate had increased by 46% compared to that of pure fuel.

Numerical simulation of single aluminum droplet evaporation based on VOF method

In order to explore the law of single aluminum droplet evaporation process in argon, based on the VOF method, this paper carried out a corresponding two-dimensional transient multiphase flow numerical simulation. The evaporation process of droplet behavior in forced convection environment is compared and analyzed, and the influences of gas velocity on evaporation rate, the change of the droplet temperature and concentration boundary layer, and the variation of the velocity field around the droplet are studied. The research results showed that droplet evaporation can be divided into transient variation period and steady-state period. As the gas velocity increased from 2 m/s to 5 m/s, the evaporation rate at the transient variation period of aluminum droplets was increased by 22%–36%, and the evaporation rate at the steady-state stage was increased by 16%–33%; At 2 m/s, the temperature boundary layer on the windward and leeward sides of the droplet was thicker than the concentration boundary layer, indicating that the temperature diffusion was greater than the concentration diffusion; In addition, when the airflow around the droplet, the double row of vortices periodically fell off from the upper and lower sides of the droplet with opposite rotation direction and regular arrangement. With the mutual interference and influence between shedding vortices, Karman vortex street was formed, and the formation of the Karman vortex street became more obvious with the increase of the Reynolds number. The research results of this paper could provide reference for the unsteady numerical simulation of aluminum droplets combustion with chemical reaction in the future.

A novel methodology to determine the specific heat capacity of lithium-ion batteries

The specific heat capacity of a battery is an essential parameter for the thermal modeling of lithium-ion batteries, but it is not generally provided by the manufacturers. To determine the specific heat capacity, equipment such as calorimeters can be utilized which is costly, whereas this paper proposed a novel method to determine the specific heat capacity that only requires conventional equipment, which is easy to find. The specific heat capacity is determined under a forced convection environment by introducing a heat loss compensate model. With this thermal compensation model, the test results wave near a constant (1044 J kg −1 ° C −1 ) regardless of the time change. To validate the method, an aluminum alloy block with the same shape as the cell is used. Results showed a specific heat capacity of 867 J kg −1 ° C −1 with a 3.3% error compared to the reference value of 897 J kg −1 ° C −1 . In addition, to verify the stability, repeated independent experiments were carried out and the results showed that this method has a good consistency. This novel methodology presents an effective way to determine the specific heat capacity at a lower cost and can be achieved quickly. • A new method to determine the heat capacity of lithium-ion batteries. • The method performs without any expensive equipment and sophisticated setups. • The method is verified by measuring the aluminum specific heat capacity. • Independent experiments were carried out with the same and different batteries.

Modeling of Forced and Natural Convection Drying Process of a Coffee Seed

HighlightsThe forced and natural convection drying processes of a single coffee grain can be predicted.The moisture distribution profiles depict the drying phenomena and water concentration zones.With accurate predictive drying curves, coffee growers can ensure high-quality coffee beans.Abstract. Different coffee drying technologies face complex tasks in ensuring an acceptable final seed moisture content. This research performed a Finite Element Analysis (FEA) study, simulating a single coffee bean's drying process as a transient mass diffusion model under mechanical and natural convection conditions, so the drying behavior and data of both case scenarios can be foreseen and controlled by a predictive Finite Element Model (FEM). A wet bean was 3D-scanned and digitized as the FEA simulation geometry; the water diffusion between the grain and the atmosphere was defined by a diffusion coefficient subject to the drying air temperature and the grain's moisture content. Three cases were studied: mechanical grain drying at three different temperatures (50, 45, and 40°C) in a forced convection environment; variable natural convection drying under environmental conditions (wet and dry season); and constant natural convection (wet and dry season), including the variation in day/night temperature and relative humidity. The results agree well with the data found in the literature, obtaining the graphical moisture distribution of the phenomena, predictive drying curves, diffusion coefficients, and isotherms. Both simulated drying scenarios provide essential information for coffee growers to improve and control their drying processes, thus obtaining high-quality grains, reducing contamination by microorganisms, and ensuring the integrity of their products. Keywords: Coffea arabica, Coffee Drying, Coffee Seed, Finite Element Model, Moisture Diffusion.

Heat Transfer Analysis of Plate Fin Heat Sink with Dimples and Protrusions: Investigation of New Designs

Numerous works have been carried out in the design and analysis of Heat Sink, however hardly any work have been reported with studies on dimples and protrusions being added to the heat sink geometries. This paper aims to find the worthiness for addition of dimples and protrusions to the common plate fin heat sink geometries for heat transfer enhancement. Analysis was carried out in forced convention environment for 9 different velocities computationally for 12 heat sink geometries at constant heat flux boundary condition applied to base of heat sink. Decrease in base temperature and thermal resistance of heat sink was considered as the performance factors. Further these computational results were validated experimentally. The results obtained from computational analysis were in good agreement with experimental results. The research outcome show that addition of dimples has proved its worthiness at slightly higher velocity regime with thermal performance being increased for a dimpled heat sink as compared to non-dimpled heat sink. Therefore, currently developed dimpled heat sinks can be a potential model that can be used in forced convection environment for better thermal performance and economy.

A Study of the Convective Cooling of Large Industrial Billets

The thermodynamic heat-transfer mechanisms, which occur as a heated billet cools in an air environment, are of clear importance in determining the rate at which a heated billet cools. However, in finite element modelling simulations, the convective heat transfer term of the heat transfer mechanisms is often reduced to simplified or guessed constants, whereas thermal conductivity and radiative emissivity are entered as detailed temperature dependent functions. As such, in both natural and forced convection environments, the fundamental physical relationships for the Nusselt number, Reynolds number, Raleigh parameter, and Grashof parameter were consulted and combined to form a fundamental relationship for the natural convective heat transfer as a temperature-dependent function. This function was calculated using values for air as found in the literature. These functions were then applied within an FE framework for a simple billet cooling model, compared against FE predictions with constant convective coefficient, and further compared with experimental data for a real steel billet cooling. The modified, temperature-dependent convective transfer coefficient displayed an improved prediction of the cooling curves in the majority of experiments, although on occasion a constant value model also produced very similar predicted cooling curves. Finally, a grain growth kinetics numerical model was implemented in order to predict how different convective models influence grain size and, as such, mechanical properties. The resulting findings could offer improved cooling rate predictions for all types of FE models for metal forming and heat treatment operations.

Experimental Investigation of spherical food product at different air velocities and radial positions during forced convection cooling

Using pre-cooling technique, food products of perishable nature are desired to be conserved from spoiling. Fruits and vegetables and are suggested to be precooled just after harvesting to avoid any food decay. The precooling enhance their storage life and keeps up quality of vegetables and fruits. Thus, an experimental analysis has been carried out to study temperature variation of spherical food product, i.e. mosambi, during the precooling process. In the current study, the effect of three different air velocities, i.e. 4.3, 4.8 and 5.0 m/s, during precooling of spherical food product has been observed in a forced convection environment. Also, determination of the thermophysical properties of the spherical food product for three different radial positions, i.e. XC= 0.042 m (centre), XM= 0.028 mm (middle) and XS= 0.014 m (surface), has been carried out. The effect of different air velocities and different radial positions on temperature profile of food product was detected. It has been noticed that proper choice of cold air velocity can decrease cooling time of spherical food product, hence resulting in energy saving.

Experimental and numerical investigation of spherical food product during forced convection cooling

Purpose of this study is to make improvements in the heat transfer behaviour during cooling of food products in forced convection environment. In the present study the effect of air velocity during pre-cooling of spherical food product i.e. orange is seen. The experiments are performed on an air blast apparatus and the results are compared with a 3D CFD model. This model is studied at (0.5, 1.0, 2.0, 2.5 and 3.0 m/s) velocities for orange. It is observed that the cooling is improved significantly up to a particular speed and increasing the speed above its critical speeds not enhance cooling significantly i.e. 2.0 m/s, hence is not advisable to increase the speed above its critical speed as it leads to loss of energy which in turn increases the overall operational cost. . The half cooling time(HCT) and the seven-eight cooling time(SECT) consequently decreases by 44.27% and 43.31% respectively for orange by a rise in air-inlet velocity from0.5 - 3.0 m/s.

Honeycomb-inspired design of a thermal management module and its mitigation effect on thermal runaway propagation

The mitigation of battery thermal runaway propagation remains challenging in the application of lithium-ion batteries, and safety enhancement remains a popular topic for battery thermal management. In this study, an aluminum honeycomb (AH) design module is proposed for a battery thermal management module, and its effects on thermal management (TM) and thermal runaway (TR) propagation are experimentally studied using an infrared imager. The results showed that AH contributed to an improved heat dissipation effect and thus mitigated thermal runaway propagation. In addition, the coupling effects of AH as well as forced convection and the phase change material (PCM) were investigated. The results indicated that the coupling effects of the AH and forced air along with the PCM both exhibited superior performance as compared to the AH alone. In a forced convection environment, the maximum temperature of the AH cell could be reduced by 17.1% under a moderate charging rate. In addition, under extreme conditions, AH was shown to mitigate TR propagation. We anticipate that the outcomes of this study can provide a new basis for the structural design of battery modules to achieve better performance and fire safety.

Comparative analysis of apple and orange during forced convection cooling: experimental and numerical investigation

The main theme of this paper is to make improvements in heat transfer during cooling of food products in forced convection environment. In the present study the effect of air velocity during precooling of spherical food products i.e., apple and orange is seen. The experiments are performed on an air blast apparatus and results are compared using simulation with a 3D CFD model. Models are developed and solved with the later one more likely to be close to experiments and hence this model is studied at 5 different velocities (0.5, 1.0, 2.0, 2.5 and 3.0 m s-1) for both samples. It is observed that cooling is improved significantly up to a particular speed for both apple and orange and increasing the speed above their respective critical speeds i.e., 2.5 m s-1 for apple and 2.0 m s-1 orange, does not enhance the cooling significantly and hence is not advisable to increase the speed above their critical speed as it leads to loss of energy which in turn increases the overall operational cost. The half cooling time and the seven-eight cooling time consequently decreases by 45.91% and 44.61%, respectively, for an individual apple with an increase in air-inflow velocity from 0.5 to 3.0 m/s and 44.27% and 43.31% respectively for orange. The food sample is rotated about its axis slowly at 2 rad s-1 and its effect is also seen. It is observed that there is significant amount of increase in the cooling rate and also the cooling is uniform.

Numerical study of methanol flames in laminar forced convective environment using short chemical kinetics mechanism

Due to recent interest in methanol economy, it is seen that a numerical study of combustion of methanol in a comprehensive manner is necessary. Motivated from this interest and based on the studies from literature, a numerical study on prediction of structures of non-premixed methanol-air flames in laminar forced convective environment is reported. Two-dimensional, planar and axisymmetric, computational domains are considered. Corresponding governing equations for conservation of mass, momentum, species and energy have been solved using Ansys FLUENT. The numerical model incorporates multi-component diffusion, variable thermal and physical properties, a short chemical kinetics mechanism with 18 species and 38 elementary reactions, and a non-luminous thermal radiation model. Homogeneous flames in opposed flow and heterogeneous flames in cross-flow and co-flow configurations are studied. For heterogeneous flames, interface conditions at the liquid methanol surface are defined systematically using a user-defined function. Numerical results are validated against the experimental results available in literature. Results in terms of mass burning rates, flow, species and temperature fields have been presented to describe the flame characteristics.

Numerical investigations of six-fold dendritic icing process in subcooled water subject to natural and forced convective environments

Six-fold symmetric ice dendrite growth in subcooled water is investigated based on a GPU-accelerated hybrid lattice Boltzmann (LB) method and the cellular automation (CA) method. A reduction factor is proposed to reduce the discretization induced anisotropy in the hybrid method. The accuracy and feasibility of this hybrid method is validated by comparing simulated tip velocity of ice dendrite with existing experimental data under microgravity conditions, as well as comparing with the LM-K theory under small degrees of subcooling. Effects of natural convection and forced convection environments on dendrite morphology and tip velocity are further studied based on this improved method. It is shown that natural convection promotes ice dendrite to grow upward, and the arm growth velocity can be enhanced or weakened by natural convection, depending on the relative angle between intrinsic growth orientation and gravity direction. Under large natural convection effects, a growth competition phenomenon between different arms on a single ice dendrite is observed which had not been predicted by previous theoretical models. On the other hand, forced convection is shown to have strong enhancing effects on upstream arms and inhibit the growth of downstream arms. The arm growth is always enhanced with increasing inlet Re to various degrees for different relative angles between intrinsic growth orientation and gravity direction.

Local Burning Rates and Heat Flux for Forced Flow Boundary-Layer Diffusion Flames

A methodology based on the Reynolds analogy was developed earlier that allowed for the estimation of local mass burning rates and heat fluxes in free-convection laminar boundary-layer diffusion flames. In this study, the relationship was examined in a forced-convective environment using methanol as a liquid fuel. The gas-phase temperature profiles across the laminar boundary layer with a methanol diffusion flame established over it were measured with the freestream air flowing parallel to the condensed fuel surface. Local and averaged mass burning rates were measured along with shear stresses at the fuel surface. The fuel consumption rate and flame lengths were observed to increase monotonically with an increase in the freestream velocity. Although the initial study was taken in the laminar regime, further extensions of the technique could be applicable to turbulent boundary-layer combustion in propulsion-oriented research.

Thermo-electrochemical analysis of lithium ion batteries for space applications using Thermal Desktop

Lithium-ion batteries (LIBs) are replacing the Nickel–Hydrogen batteries used on the International Space Station (ISS). Knowing that LIB efficiency and survivability are greatly influenced by temperature, this study focuses on the thermo-electrochemical analysis of LIBs in space orbit. Current finite element modeling software allows for advanced simulation of the thermo-electrochemical processes; however the heat transfer simulation capabilities of said software suites do not allow for the extreme complexities of orbital-space environments like those experienced by the ISS. In this study, we have coupled the existing thermo-electrochemical models representing heat generation in LIBs during discharge cycles with specialized orbital-thermal software, Thermal Desktop (TD). Our model's parameters were obtained from a previous thermo-electrochemical model of a 185 Amp-Hour (Ah) LIB with 1–3 C (C) discharge cycles for both forced and natural convection environments at 300 K. Our TD model successfully simulates the temperature vs. depth-of-discharge (DOD) profiles and temperature ranges for all discharge and convection variations with minimal deviation through the programming of FORTRAN logic representing each variable as a function of relationship to DOD. Multiple parametrics were considered in a second and third set of cases whose results display vital data in advancing our understanding of accurate thermal modeling of LIBs.

Investigation of Thermal Characterization of a Thermally Enhanced FC-PBGA Assembly

In this paper, three-dimensional finite element analysis using the commercial ANSYS software is performed to study the thermal performance of a thermally enhanced FC-PBGA (flip-chip plastic ball grid array) assembly in both natural and forced convection environments. The thermally enhanced FC-PBGA assembly is a basic FC-PBGA assembly with a lid attached on top, after which an extruded-fin heatsink is attached on the top of the lid. The finite element model is complete enough to include key elements such as bumps, solder balls, substrate, printed circuit board, extruded-fin heatsink, lid, vias, TIM1 (thermal interface material 1), TIM2 (thermal interface material 2), lid-substrate adhesive and ground planes for both signal and power. Temperature fields are simulated and presented for several package configurations. Thermal resistance is calculated to characterize and compare the thermal performance by considering alternative design parameters of the polymer-based materials and the thermal enhancement components. The polymer-based materials include underfill, TIM1, TIM2, lid-substrate adhesive and substrate core material. The specific thermal enhancement components are the extruded-fin heatsink and the lid.

Numerical study of Marangoni convection during transient evaporation of two-component droplet under forced convective environment

Numerical simulations of the evaporation of stationary, spherical, two-component liquid droplets in a laminar, atmospheric pressure, forced convective hot-air environment are presented. The transient two-phase numerical model includes multi-component diffusion, a comprehensive method to deal with the interface including the surface tension effects and variation of thermo-physical properties as a function of temperature and species concentration in both liquid- and vapor-phases. The model has been validated using the experimental data available in literature for suspended heptane–decane blended droplets evaporating under a forced convective air environment. The validated model is used to study the vaporization characteristics of heptane–decane droplets under different convective conditions. For an initial composition having 75% by volume of more volatile fuel component, the evaporation transients are presented in terms of variations in interface quantities. Flow, species and temperature fields are presented at several time instants to show the relative strengths of forced convection and Marangoni convection. Results show that at low initial Reynolds numbers, the solutal Marangoni effects induce a flow-field within the liquid droplet, which opposes the flow of the external convective field. The strength of this liquid-phase flow field increases with the consumption of the more volatile fuel component.

Numerical Study of Thermal Performance of FC-PBGA Assembly with Extruded-fin Heatsink

The trend in microelectronics is toward increasingly higher input/output, higher component density and higher electrical performance, which makes thermal enhancement of package performance an increasingly important issue. In both natural and forced convection environments, three-dimensional finite element simulation is used to study thermal performance of a flip-chip plastic ball grid array (FCPBGA) assembly with an extruded-fin heatsink on top of the assembly. The finite element model is complete enough to include key elements such as bumps, solder balls, substrate, printed circuit board, vias and ground planes for both signal and power. Temperature fields are simulated and presented for several FC-PBGA assembly configurations. Thermal resistance is calculated to characterize and compare the thermal performance by considering alternative design parameters of the extruded-fin heatsink and the lid.

Thermal Design and Analysis of Electronic Enclosure of Ground Radar

When an electronic component is powered heat will be developed which will in turn increases the component temperature. For safe functioning of these components manufacturer specifies allowable temperature. If the component temperature exceeds these limits component fails. To get rid of this situation heat dissipation mechanism must be effective. Thermal design of an enclosure for PCBs of a ground RADAR is taken up in this work. The primary objective is to enhance the heat dissipation rate with the proposed enclosure in forced convection environment. Selection of fan and estimation of cooling rate is also be done. The complexity associated with this requirement is to estimate the heat transfer coefficient for the proposed enclosure in forced convection environment. This is due to the reason that the standard literature limits its scope to presentation of empirical correlations for heat transfer coefficients for standard geometries like plate, pipe, etc. Where as in the current requirement heat transfer coefficient is to be estimated for the enclosure with complex geometry. As part of the work an analytical method is established to estimate the heat transfer coefficient and the same is validated with commercial CFD software package. After validation process maximum component temperature is calculated in order to ascertain the cooling effectiveness of the proposed thermal design by ensuring that maximum temperature is within the prescribed limits. Apart from meeting the primary objective i.e. providing suitable heat dissipation mechanism and ensuring component temperature within limits another intended outcome of the work is an analytical method to calculate heat transfer coefficient for electronic enclosures quickly without depending on commercial CFD software.

Piloted ignition delay times of opposed and concurrent flame spread over a thermally-thin fuel in a forced convective microgravity environment

Piloted ignition delay of thermally-thin cellulose as a function of forced flow, oxygen concentration, and pressure was studied in a low-speed flow tunnel mounted on a NASA Zero Gravity Research Facility drop vehicle. Fuel heat-up times are very short and invariant due to the rapid heating of the pilot igniter and thermally-thin fuel used in this study. The effect of flow was found to be negligible for the test conditions studied, where mixing times were very rapid. Ignition delay in these tests was found to be dominated by the induction time, which was found to be inversely proportional to the oxygen concentration and the total pressure, or equivalently, the oxygen partial pressure. The reaction rate (the inverse of the induction time) is thus linear with the oxygen partial pressure, in agreement with kinetic theory for simple thermal reactions.