Free Convection Effects Research Articles

LPG vessels exposed to fire: Scale effects on pressure build-up

Fires may induce the thermal failure of LPG storage and transport vessels, leading to severe escalation scenarios. In the present study, an innovative CFD modeling approach was developed to investigate the thermal behavior of LPG tanks exposed to fires to support the prevention of catastrophic scenarios. For the first time, several data-sets were used to assess the model predictions considering different vessel scales, fire geometries and configurations. Comparison with experimental results demonstrates the predictive capabilities of the approach. The model allowed analyzing the role of different key phenomena, such as: the effect of free convection and thermal boundary layer forming in the near wall region, the thermal stratification, both in the liquid and the vapor phases, and the liquid thermal expansion. In addition, the influence of the vessel geometry on such phenomena was investigated. The results obtained may drive the design of emergency response, improving the safety of hazardous materials storage and transport systems.

CFD study of the supercritical transesterification of triolein: Wall-to-fluid/particle-to fluid transport effects

Transesterification of triglycerides is a process by which biodiesel is produced. Biodiesel is a sustainable alternative to petroleum diesel; hence, its efficient production is of significant importance. The transesterification of triolein with methanol under supercritical conditions in the presence of a solid catalyst structured as packed beds is of particular interest due to high yields at moderate temperatures and continuous operability. Fundamental understanding of the heat and mass transfer phenomena in a supercritical transesterification reaction is paramount for a better modelling of the reaction.Computational Fluid Dynamics (CFD) were used to study, by means of a simplified numerical model, heat and mass transport phenomena in a supercritical transesterification reactor. The CFD model was built replicating an experimental pilot plant reactor. Simulations were performed for a 3D geometrical model of a packed bed of cylindrical catalysts. Wall-to-fluid and particle-to-fluid heat and mass transfer were studied for a fluid mixture of methanol (CH3OH) and triolein (C57H104O6) along with co-solvent carbon dioxide (CO2), flown into the packed bed under supercritical conditions (25 MPa, 473.15 K). Detailed flow, temperature and concentration profiles in packed bed were obtained. Free and forced convection effects for different flow rates were analyzed. Large density gradients were observed near the packed bed inlet, generating hydrodynamic instabilities due to natural convection as observed in Metais-Eckert maps. Heat and mass transfer coefficients were estimated from CFD data, and the obtained values of Nuw, Nup, Shp were compared against empirical models for different flow rates for validation of the CFD data. The simulation data agreed well with empirical correlationsfor predicting transport coefficients.

Mixed convection and thermodynamic irreversibilities in MHD nanofluid stagnation-point flows over a cylinder embedded in porous media

The impingement of CuO-water nanofluid flows upon a cylinder subject to a uniform magnetic field with constant surface temperature and embedded in porous media is investigated for the first time in literature. The surface of the cylinder can feature uniform or non-uniform mass transpiration and is hotter than the incoming nanofluid flow. The gravitational effects are taken into account and the three-dimensional governing equations of mixed convection in curved porous media, under magnetohydrodynamic effects, are reduced to those solvable by a finite difference scheme. Through varying a mixed convection parameter, the situations dominated by forced, mixed and free convection are examined systematically. The numerical solutions of these equations reveal the flow velocity and temperature fields as well as the Nusselt number and induced shear stress. These are then used to calculate the rate of entropy generation within the system by viscous and heat transfer irreversibilities. The results show that Nusselt number increases with increasing the concentration of nanoparticles, while it slightly deceases through intensifying the magnetic parameter. Non-uniform transpiration is shown to strongly affect the average rate of heat transfer. Importantly, it is demonstrated that the specific mode of heat convection can majorly influence the intensity of entropy generation and that the irreversibilities are much larger under natural convection compared to those in mixed and forced convection. Calculation of Bejan number shows that this is due to more pronounced relative contribution of viscous irreversibilities when free convection effects dominate the mixed convection process.

Improved Characterization of Groundwater Flow in Heterogeneous Aquifers Using Granular Polyacrylamide (PAM) Gel as Temporary Grout

AbstractThe range of options for investigation of hydraulic behavior of aquifers from boreholes has been limited to rigid, cumbersome packers, and inflatable sleeves. Here we show how a new temporary borehole sealing technique using soft grains of polyacrylamide (PAM) gel as a sealing material can be used to investigate natural groundwater flow dynamics and discuss other possible applications of the technology. If no compressive stress is applied, the gel packing, with a permeability similar to open gravel, suppresses free convection, allowing for local temperature measurements and chemical sampling through free‐flowing gel packing. Active heating laboratory and field experiments combined with temperature measurements along fiber optic cables were conducted in water‐filled boreholes and boreholes filled with soft grains of polyacrylamide gel. The gel packing is shown to minimize the effect of free convection within the well column and enable detection of thin zones of relatively high or low velocity in a highly transmissive alluvial aquifer, thus providing a significant improvement compared to temperature measurements in open boreholes. Laboratory experiments demonstrate that under modest compressive stress to the gel media the permeability transitions from highly permeable to nearly impermeable grouting. Under this configuration the gel packing could potentially allow for monitoring local response pressure from the formation with all other locations in the borehole hydraulically isolated.

Inlet tube spacing and protrusion inlet effects on multiple circular tubes in the laminar, transitional and turbulent flow regimes

The purpose of this study was to investigate inlet tube spacing and protrusion effects on multiple circular tubes in the laminar, transitional and turbulent flow regimes. An experimental set-up was built for this investigation and three configurations of test sections were investigated. The first was a single-tube test section for validation purposes, of which the results were compared with literature. The second was two multi-tube test sections with three tubes spaced at different pitches. The third configuration was similar to configuration two, except that the centre tube had a small protrusion. All the tubes had an inner diameter of 3.97 mm, and long tube lengths of 6 m were used to ensure fully developed flow. The tubes were electrically heated that ensured a constant heat flux heating condition. Water was used as the test fluid, and the Prandtl number varied between 3 and 7. The experiments were conducted at heat fluxes of 2, 3 and 4 kW/m2 for Reynolds numbers between 1000 and 7000, to ensure that the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes, were covered. The tubes were spaced apart from each other at 1.25, 1.4 and 1.5 times the outer tube diameter, and the protrusion of the centre tube was 10% of the tube inner diameter. It was found that an increased pitch ratio dampened the inlet disturbances in the centre tube and reduced the flow asymmetry in the side tubes, therefore the differences in the critical Reynolds numbers and transition gradients of the three tubes decreased. As the inlet disturbances were damped in the centre tube, transitional was delayed compared to a single tube with a square-edged inlet. For the side tubes, the increased flow asymmetry led to increased critical Reynolds numbers, as well as increased transition gradients. The presence of a protrusion inlet in the centre tube significantly increased the asymmetry of the flow in the side tubes, which led to an additional increase in the critical Reynolds numbers and the transition gradients increased. Free convection effects also led to increased critical Reynolds numbers and transition gradients, as well as decreased differences between the results of the tubes in the multi-tube set-up when a square-edged inlet was used. However, free convection effects were not able to dampen the inlet disturbances caused by a protrusion inlet in the centre tube.

Heat transfer of developing and fully developed flow in smooth horizontal tubes in the transitional flow regime

Limited experimental work has been done specifically focussing on how the heat transfer characteristics of developing flow in the transitional flow regime changes along the tube length, and how it compares with fully developed flow. The purpose of this study was therefore to experimentally investigate the heat transfer characteristics of developing and fully developed flow in smooth horizontal tubes in the transitional flow regime at a constant heat flux. An experimental set-up was designed, built and validated against literature. Two smooth circular test sections with inner diameters of 4mm and 11.5mm, respectively, were used and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 700 and 10000 at different heat fluxes. The boundaries of the different flow regimes were defined mathematically, and terminology to define the transition characteristics were presented. It was found that the Reynolds number at which transition started was independent of axial position, and transition occurred at the same moment in time along the whole tube length. However, the end of transition was dependent on axial position and occurred earlier as the flow approached fully developed flow. Free convection effects affected both the start and end of the transitional flow regime, and caused the Reynolds number range of the transitional flow regime to decrease. Correlations were developed to determine the start and end of the transitional flow regime for developing and fully developed flow for mixed convection conditions. The transitional flow regime of developing flow was divided into three regions. In the first region, the width of the transitional flow regime decreased significantly with axial position and free convection effects were negligible. In region 2, the width of the transitional flow regime decreased with both axial position and free convection effects. In the fully developed region, the width of the transitional flow regime was independent of axial position, but free convection effects caused it to decrease and even to become negligible. It was concluded that the heat transfer characteristics of developing flow in the transitional flow regime changes as the flow develops and differ significantly from fully developed flow.

Single-phase mixed convection of developing and fully developed flow in smooth horizontal circular tubes in the laminar and transitional flow regimes

Correlations to calculate the local and average heat transfer coefficients for single-phase laminar flow in horizontal circular tubes with a constant heat flux boundary condition are usually restricted to fully developed flow, high Prandtl numbers or constant fluid properties. What further complicates the heat transfer characteristics of developing flow, is the local transition from laminar to turbulent flow along the tube length, above the critical Reynolds number. The purpose of this study was to investigate the effects of free convection on the development of the local heat transfer characteristics in smooth horizontal circular tubes heated with a constant heat flux. An experimental set-up was designed, built, and results were validated against literature. Two smooth circular test sections with inner diameters of 4mm and 11.5 mm were used, and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 500 and 10,000 at different heat fluxes. A total of 1046 mass flow rate measurements and 89,459 temperature measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. Three different regions were identified for developing laminar flow and were qualitatively and quantitatively defined. Correlations were developed to determine the thermal entrance lengths, as well as local and average Nusselt numbers for developing and fully developed laminar flow in mixed convection conditions with a constant heat flux boundary condition. In the transitional flow regime, the laminar-turbulent transition along the tube length was divided into four regions, and it was found that the flow transitioned faster with increasing free convection effects and Reynolds number.

Flow regime maps for smooth horizontal tubes at a constant heat flux

The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl number fluids, however, they are inaccurate at lower Prandtl numbers and tube diameters and specifically for water as the working fluid. The purpose of this paper was to develop a flow regime map that can be used for both high and low Prandtl number fluids for a wide range of tube diameters. Furthermore, not only to develop a map for fully developed flow, but also for developing flow. An experimental set-up was designed, built and validated against results from literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively, and lengths of 5.5 m and 9.5 m, respectively, were used. Heat transfer measurements were taken at a range of Reynolds numbers from laminar to turbulent. Water was used as the test fluid and the Prandtl number was limited between 3 and 7. Experimental results from literature complemented the test matrix used for this study, especially with high Prandtl number data. Six flow regime maps were developed for a constant heat flux boundary condition and a square-edged inlet, and these flow regime maps are original for four reasons. Firstly, it contains contour lines that show the Nusselt number enhancements due to free convection effects. Secondly, it is valid for a wide range of tube diameters and Prandtl numbers. Thirdly, the flow regime maps were developed as a function of temperature difference (Grashof number) and heat flux (modified Grashof number). Finally, four of the six flow regime maps are not only valid for fully developed flow, but also developing flow.

Unsteady free convection oscillatory couette flow through a variable porous medium with concentration profile

In this paper we have studied the effect of free convection on the heat transfer and flow through variable porous medium which is bounded by two vertical parallel porous plates. In this study it is assume that free stream velocity oscillates with time about a constant mean. Periodic temperature is considered in the moving plate. Effect of different parameters on mean flow velocity, Transient velocity, Concentration profile and transient temperature studied in detail.

Mixed convection in turbulent channels with unstable stratification

We study turbulent flows in pressure-driven planar channels with imposed unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of fully developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi-streamwise rollers occupying the full channel thickness, with aspect ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum, providing for a substantial fraction of the heat and momentum flux at bulk Richardson numbers larger than$0.01$. The mean values and the variances of the flow variables do not appear to follow Prandtl’s scaling in the free-convection regime, except for the temperature and vertical velocity fluctuations, which are more directly affected by wall-attached turbulent plumes. We find that the Monin–Obukhov theory nevertheless yields a useful representation of the main flow features. In particular, the widely used Businger–Dyer flux-profile relationships are found to provide a convenient way of approximately accounting for the bulk effects of friction and buoyancy, although the individual profiles may have wide scatter from the alleged trends. Significant deviations are found in direct numerical simulations with respect to the commonly used parametrization of the momentum flux in the light-wind regime, which may have important practical impact in wall models of atmospheric dynamics. Finally, for modelling purposes, we devise a set of empirical predictive formulae for the heat flux and friction coefficients, which are within approximately$10\,\%$standard deviation from the numerical results in a wide range of flow parameters.

Mixed convection heat transfer: an experimental study on Cu/heat transfer oil nanofluids inside annular tube

The heat transfer and flow characteristics of Cu–heat transfer oil nanofluid during mixed convection through horizontal annular tubes under uniform heat flux as boundary condition are investigated experimentally. Data were acquired at low Reynolds number ranged from about 26 to 252. The applied nanofluid prepared by Electrical Explosion of Wire technique with no nanoparticles agglomeration during nanofluid preparation process and experiments. Pure heat transfer oil and nanofluids with nanoparticles weight concentrations of 0.12, 0.36 and 0.72% were used as the working fluids. Based on these results, Effects of nanoparticles concentration, heat flux and free convection on the thermal field development are studied under buoyancy assisted flow condition for Grashof number, Richardson number between 2820 and 12,686, and 0.1–10, respectively. Results show that Nusselt number increases with an increase of nanoparticles weight concentrations from 0 to 0.72% under certain Richardson numbers.

Possible Similarity Cases for Internal Heat Generation, Thermal Radiation and Free Convection of Unsteady Boundary Layer Flow Over a Vertical Plate

In this paper we analyze the effects of free convection for internal heat generation and thermal radiation of an unsteady boundary layer flow over a vertical plate. We have applied free parameter method to obtain possible similarity cases. One of these cases have been investigated analytically and numerically. In this case, the governing non-linear partial differential equations are transformed into a system of coupled non-linear ordinary differential equations using similarity transformations and then solved numerically using the Runge–Kutta method with shooting technique for better accuracy. The effects of the governing parameters on the flow and thermal fields are investigated numerically and displayed graphically. The skin-friction coefficient, heat transfer coefficient have also been obtained and presented in tabular form.

Chemical reaction and Soret effects on hydromagnetic micropolar fluid along a stretching sheet

Free convection effects of a micropolar fluid along a stretching sheet embedded in a porous medium in the presence of a volumetric non-uniform heat source is investigated in the present paper. Thermal diffusion and first order chemical reaction are also considered in the present study to govern the flow characteristic. The generalization of the earlier studies centers round: (i) The magnetohydrodynamic flow is made to pass through a porous medium characterized by a non-Darcian drag coefficient affecting the momentum equation. (ii) The energy equation is modified with the interplay of non-uniform heat source. (iii) Consideration of chemically reactive species characterized by first order chemical reaction and thermal diffusion i.e. Soret modifying the equation of species concentration. Similarity transformation technique is used to transform the governing nonlinear partial differential equations into ordinary differential equations. The numerical solutions are achieved showing the effects of pertinent parameters. For verification of the present findings the results of this study have been compared with the earlier works in particular cases.

Free convection effects on a vertical cone with variable viscosity and thermal conductivity

The present paper deals with a flow of a viscous incompressible fluid along a heated vertical cone, with due allowance for variations of viscosity and thermal diffusivity with temperature. The fluid viscosity is assumed to be an exponential function of temperature, and the thermal diffusivity is assumed to be a linear function of temperature. The governing equations for laminar free convection of the fluid are transformed into dimensionless partial differential equations, which are solved by a finite difference method with the Crank–Nicolson implicit scheme. Dependences of the flow parameters on the fluid viscosity and thermal conductivity are obtained.

Magnetohydrodynamics Mixed Convection in a Lid-Driven Cavity Having a Corrugated Bottom Wall and Filled With a Non-Newtonian Power-Law Fluid Under the Influence of an Inclined Magnetic Field

In this study, the problem of magnetohydrodynamics (MHD) mixed convection of lid-driven cavity with a triangular-wave shaped corrugated bottom wall filled with a non-Newtonian power-law fluid is numerically studied. The bottom corrugated wall of the cavity is heated and the top moving wall is kept at a constant lower temperature while the vertical walls of the enclosure are considered to be adiabatic. The governing equations are solved by the Galerkin weighted residual finite element formulation. The influence of the Richardson number (between 0.01 and 100), Hartmann number (between 0 and 50), inclination angle of the magnetic field (between 0 deg and 90 deg), and the power-law index (between 0.6 and 1.4) on the fluid flow and heat transfer characteristics are numerically investigated. It is observed that the effects of free convection are more pronounced for a shear-thinning fluid and the buoyancy force is weaker for the dilatant fluid flow compared to that of the Newtonian fluid. The averaged heat transfer decreases with increasing values of the Richardson number and enhancement is more effective for a shear-thickening fluid. At the highest value of the Hartmann number, the averaged heat transfer is the lowest for a pseudoplastic fluid. As the inclination angle of the magnetic field increases, the averaged Nusselt number generally enhances.

Free Convection Effects on a Vertical Cone with Variable Viscosity and Thermal Conductivity

Free Convection Effects on a Vertical Cone with Variable Viscosity and Thermal Conductivity

MONTE CARLO BASED UNCERTAINTY ANALYSIS FOR VARIABLE PROPERTY MIXED CONVECTION FLOW IN A UNIFORMLY HEATED CIRCULAR TUBE

A Monte Carlo based stochastic numerical analysis of simultaneously developing mixed convection laminar flow and heat transfer with variable properties in a uniformly heated circular tube was carried out. The Monte Carlo simulations were performed for heating of a large Prandtl number liquid for a wide range of Prandtl numbers (50 ≤ Pr ≤ 1000), Rayleigh numbers (105 ≤ Ra ≤ 107), and nondimensional heat flux values (100 ≤ qwd/k ≤ 1000). The results show that the effect of uncertainties in the properties is greater on the friction factors uncertainty than it is on the Nusselt numbers uncertainty, and the uncertainty of the predicted friction factors and Nusselt numbers increases with increasing free convection and entrance effects. The maximum uncertainties in the predicted friction factors and Nusselt numbers were around 7.5% and 3.3%, respectively, for a 10% uncertainty value in the fluid properties. The uncertainties in the friction factors and Nusselt numbers were found to increase approximately linearly with increasing fluid property uncertainties.

Computational thermal analysis of cylindrical fin design parameters and a new methodology for defining fin structure in LED automobile headlamp cooling applications

In this study, the effects of fin design, fin material, and free and forced convection on junction temperature in automotive headlamp cooling applications of LED lights are researched by using ANSYS CFX 14 software. Furthermore a new methodology is presented for defining the optimum cylindrical fin structure within the given limits. For measuring the performance of methodology, analyses are carried out for various ambient temperatures (25 °C, 50 °C and 80 °C) and different LED power dissipations (0.5 W, 0.75 W, 1 W and 1.25 W). Then, analyses are repeated at different heat transfer coefficients and different fin materials in order to calculate LED junction temperature in order to see if the fin structure proposed by the methodology is appropriate for staying below the given safety temperature limit. As a result, the suggested method has always proposed proper fin structures with optimum characteristics for given LED designs. As another result, for safe junction temperature ranges, it is seen that for all LED power dissipations, adding aluminum or copper plate behind the printed circuit board at low ambient temperatures is sufficient. Also, as the ambient temperature increases, especially in high powered LED lights, addition of aluminum is not sufficient and fin usage becomes essential. High heat transfer coefficient and using copper fin affect the junction temperature positively.

Quantification of free convection effects on 1 kg mass standards

We determine the free-convection effects and the resulting mass differences in a high-precision mass comparator for cylindrical and spherical 1 kg mass standards at different air pressures. The temperature differences are chosen in the millikelvin range and lead to microgram updrafts. Our studies reveal a good agreement between the measurements and direct numerical simulations of the Boussinesq equations of free thermal convection. A higher sensitivity to the free convection effects is found for the spherical case compared to the cylindrical one. We also translate our results on the free convection effects into a form which is used in fluid mechanics: a dimensionless updraft coefficient as a function of the dimensionless Grashof number Gr that quantifies the thermal driving due to temperature differences. This relation displays a unique scaling behavior over nearly four decades in Gr and levels off into geometry-specific constants for the very small Grashof numbers. The obtained results provide a rational framework for estimating systematic errors in mass metrology due to the effects of free convection.

Numerical investigation on the temperature control of a NIF cryogenic target

Numerical investigation was performed on the temperature control of NIF cryogenic target in order to get a temperature uniformity of 0.1mK on the surface of the capsule. Heat transfer process was discussed to find out major factors in the temperature control, tamping gas heat transfer and free convection of the tamping gas was calculated. Spherically symmetric temperature field is required due to energy released from the tritium decay within the capsule, auxiliary heating is set on the hohlraum to compensate the higher heat loss caused by the lower tamping gas thermal resistance on the mid plane. Free convection effect of the tamping gas is reduced by separating the tamping gas with plastic films and independent temperature control of the cooling arm. This research may provide theoretical foundation and reference for temperature control on the cryogenic target.