Constant Surface Temperature Research Articles

Physical Mechanisms Controlling the Offshore Propagation of Convection in the Tropics: 1. Flat Island

AbstractAn idealized convection‐permitting simulation is performed to investigate the physical mechanisms responsible for the nighttime offshore propagation of convection around tropical islands. An idealized island is placed in the middle of a long, nonrotating channel oceanic domain with constant sea surface temperature. A strong diurnal cycle typical of a tropical island is simulated, with a thermally forced sea breeze in daytime and the associated inland propagation of precipitation. Offshore propagation of a land breeze and its associated convection is simulated every night but with varying extent. Gravity waves of first and second baroclinic modes trigger convection far from the coast if the offshore conditions are favorable. This accelerates the propagation speed of the land breeze as it reduces the onshore wind associated with the lower branch of the overturning large‐scale circulation. Higher‐order modes may trigger convection or reinforce existing convection but less systematically. The distance of propagation is particularly sensitive to humidity and temperature at the top of the boundary layer, with occasional incursions of a dry anomaly at the top of the boundary layer near the island preventing convection from developing far from the island.

Maximization of heat transfer density from a vertical array of flat tubes in cross flow under fixed pressure drop using constructal design

AbstractThe density of heat transfer rate from a vertical array of flat tubes in cross flow is maximized under fixed pressure drop using constructal design. With the constructal design, the tube arrangement is found such that the heat currents from the tubes to the coolant flow easily. The constraint in the present constructal design is the volume where the tubes are arranged inside it. The two degrees of freedom available inside the volume are the tube‐to‐tube spacing and the length of the flat part of the tubes (tube flatness). The tubes are heated with constant surface temperature. The equations of continuity, momentums, and energy for steady, two‐dimensional, and laminar forced convection are solved by means of a finite‐volume method. The ranges of the present study are Bejan number (dimensionless pressure drop) (103 ≤ Be ≤ 105) and tube flatness (dimensionless length of the tube flat part) (0 ≤ F ≤ 0.8). The coolant used is air with Prandtl number (Pr = 0.72). The results reveal that the maximum heat transfer density decreases when the tube flatness decreases at constant Bejan number. At constant tube flatness, the heat transfer density increases as the dimensionless pressure drop (Bejan number) increases. Also, the optimal tube‐to‐tube spacing is constant, irrespective of the tube flatness at constant Bejan number.

Improved analytical prediction of ground frost heave during tunnel construction using artificial ground freezing technique

The reasonable prediction of ground frost heave provides guiding significance for tunnel construction that uses artificial ground freezing technique. Single-pipe freezing theory, which considers the freezing point of soil and assumes the constant surface temperature of a freezing pipe, is suggested for solving the radius of the freezing front before the closure of the frozen wall. By contrast, the radius of the freezing front after the closure of the frozen wall can be calculated using flat-panel freezing theory, which considers the freezing point of soil and determines the average temperature of the freezing pipe circle. On the basis of these suggestions and the analytical prediction proposed by Cai et al. (2014), an improved analytical prediction of ground frost heave was established by the stochastic medium theory according to the formation process of the frozen wall. The improved analytical prediction was applied to an actual tunnel freezing project. Then, the heaving and horizontal displacements of the ground surface are obtained, which agree well with the field-measured data. The effectiveness and practicality of the improved analytical prediction are verified in this study.

Effects of Temperature and Humidity on the Skin Permeation of Hydrophilic and Hydrophobic Drugs.

The humidity was a well-known method to hydrate the skin; however, the published data were varied, and systemic experiments in the previous papers were few. Therefore, the in vitro permeation of excised porcine ear skin by drugs with different polarities [aminopyrine (AMP), antipyrine (ANP), methylparaben (MP), and ibuprofen (IP)] was analyzed under a constant skin surface temperature with different temperatures and humidities to reveal the effects of temperature and humidity on the skin permeation enhancement effects. Applied formulations were prepared by mixing the drug and a hydrophilic vehicle containing glycerin. The disposition-distance profiles of water and the humectant glycerin in the stratum corneum were also investigated using confocal Raman microscopy. High absolute humidity (AH) significantly contributed to the high skin penetration of the hydrophilic penetrants AMP, ANP, and MP but not the hydrophobic penetrant IP. An increase in the partition parameter and a decrease in the diffusivity parameter occurred with an increase in AH, independent of drug polarity. Moreover, we found that dew condensation induced by high AH on temperature-controlled skin surface may effectively increase water content and may provide higher glycerin distribution in the skin barrier, the stratum corneum. Increasing the amount of water and hydrophilic vehicles such as glycerin in the stratum corneum may enhance the permeation of hydrophilic penetrants AMP, ANP, and MP. These data suggested a dew condensation on the skin surface induced by high AH at a constant skin surface temperature would be important to enhance hydrophilic penetrants.

An MHD viscous liquid stagnation point flow and heat transfer with thermal radiation and transpiration

The impact of kinematic parameters magnetohydrodynamic (MHD) and thermal radiation on the unsteady flow of a Newtonian liquid through stagnation point due to a linear sheet with mass transpiration is considered. The characteristics of heat and MHD impinging on the sheet are analyzed theoretically. The flow of an electrically conducting liquid through stagnation point has gained considerable interest due to its industrial relevance. In the chemical engineering applications involving cooling of the liquid namely glass blowing, food processing, metal thinning, polymer extrusion, silicon chip manufacturing and applications of similar kind. In all these chemical engineering applications, the interplay between the regulating kinematic parameters and the nature of the fluid is of at most priority. The flow problem is modelled into nonlinear unsteady Navier-Stokes’ partial differential equations. The similarity solution for the velocity distribution is obtained. Depending on the type of boundary heating, the analytical solutions for temperature distribution is derived by means of a power series (Gauss hypergeometric). Temperature distribution for two types of boundary heating processes viz., prescribed time-dependent constant surface temperature (PTDCST) and prescribed time-dependent wall heat flux (PTDWHF) is discussed. There found to exist branching of solutions for both velocity and temperature distribution for certain range of controlling parameters. In fact there exists dual solution for both cases of stretching/shrinking sheet and these are analyzed to see the impacts of various physical parameters on the solution domain. The impact of various regulating parameters on the velocity as well as temperature is analyzed by means of numerous plots.

Numerical investigation on the heat transfer characteristics of liquid-liquid plug-train in microchannels

Liquid-liquid plug-train is of great importance due to its wide applications in the heat transfer enhancement and the richness of underlying physics. This paper presented a theoretical and numerical study of the liquid-liquid plug-train in a microchannel. Based on the analytical model of flow field, the heat transfer process in liquid-liquid plug-train was investigated. The constant surface temperature boundary condition was considered. Nusselt number (Nu) and heat transfer index (η) were employed to evaluate the heat transfer characteristics of liquid-liquid plug-train in the microchannel. Three stages of the heat transfer process in liquid-liquid plug-train were identified: (i) development of thermal boundary layer (TBL); (ii) advection of heated/fresh fluid in the liquid-liquid plug train; and (iii) thermally fully developed flow. The effects of the thermal conductivity ratio and the plug length ratio were investigated. A low plug length ratio was found to be effective in heat transfer enhancement in the microchannel.

Oxygen atom kinetics in CO2 plasmas ignited in a DC glow discharge

Oxygen atom densities were measured in situ in a CO2 glow discharge, at pressures between 0.2 and 5 Torr (26.7–666.6 Pa). Two measurement techniques were compared, namely optical emission actinometry (using Ar as the actinometer) and high-resolution two-photon absorption laser induced fluorescence normalised to Xe, and were found to give consistent results. The variation of the atomic oxygen density with gas pressure shows two different regimes with a transition around 1 Torr. Measurements of the O atom loss frequency under plasma exposure showed that this behaviour is caused by a change in the O atom loss mechanisms, which are dominated by surface processes in our experimental conditions. The corresponding recombination probabilities on Pyrex γO are found to vary with the gas temperature near the wall for a constant surface temperature, similarly to what has recently been obtained in pure O2. However, the measured values are more than two times lower than γO obtained in a O2 plasma in similar conditions. The O atom densities are also compared to the dissociation fraction of CO2 determined by infra-red absorption. The obtained CO and O densities show different behaviour as a function of the energy input. The simultaneous measurement of gas temperature, electric field, O, CO and CO2 densities and O atoms loss frequency in the same conditions provides an ideal set of constraints for validating CO2 plasma kinetic models.

Natural Convection Flow of Power-Law Fluids over a Heated Horizontal Plate Surface

Abstract The current work attempts to investigate the existence of similarity solution for the natural convection boundary layer flow of a power-law fluid over a heated horizontal semi-infinite plate surface. Three types of surface temperature variation are considered, namely (i) constant surface temperature (CST), (ii) power-law variation of surface temperature (PST) and (iii) power-law variation of surface heat flux (PHF). A reduction of the governing partial differential equations is achieved by using a suitable similarity transformation. But, a careful mathematical verification confirms that the similarity solution does not exist in the CST case and for the PST case it exists only under a particular value of the wall temperature parameter r = 1/3. It also ensured the existence of similarity solution for the PHF case and this will happen only when the plate surface is held at a constant heat flux situation. A new fundamental parameter S arises from the dimensionless analysis which is reduced to the reciprocal of the usual Prandtl number Pr for Newtonian fluids. A novel result of this analysis is that the salient features of the Grashof number Gr which differentiates the thermal responses of the shear-thinning fluid (0 < n < 1) flows from the shear-thickening fluid (n > 1) flows while the Newtonian fluid (n = 1) flows remain unaffected by this parameter no matter what be the value of S.

Viscous Dissipation Effects in A Microchannel Caused by Oscillation of One Surface

The viscous dissipation effects in a microchannels caused by an oscillatory lower surface is investigated numerically. An asymmetric thermal boundary condition, particularly at upper plate insulated and lower plate with constant surface temperature is solved and analyzed in details graphically. Results reveal that effect of temperature field is strongly dependent on Brinkman number, while the thermal diffusion rate on the heat induced relies on the Prandtl number. The angular frequency has influence on the temperature field gradient.

Viscous Dissipation Effects in A Microchannel Caused by Oscillation of One Surface

The viscous dissipation effects in a microchannels caused by an oscillatory lower surface is investigated numerically. An asymmetric thermal boundary condition, particularly at upper plate insulated and lower plate with constant surface temperature is solved and analyzed in details graphically. Results reveal that effect of temperature field is strongly dependent on Brinkman number, while the thermal diffusion rate on the heat induced relies on the Prandtl number. The angular frequency has influence on the temperature field gradient.

A Theoretical Study of Steady MHD mixed convection heat transfer flow for a horizontal circular cylinder embedded in a micropolar Casson fluid with thermal radiation

In this study, an investigation is carried out for laminar steady mixed 2D magnetohydrodynamic (MHD) flow of micropolar Casson fluid with thermal radiation over a horizontal circular cylinder with constant surface temperature. In the present study, an investigation is carried out on the effects of physical parameters on Casson fluid non dimensional numbers. The governing nonlinear partial differential equations and the controlling boundary conditions are derived for this case study. Furthermore, these equations are solved numerically using finite difference technique known as Keller Box Method (KBM). The effects of non-dimensional governing parameters, namely Casson parameter, mixed convection parameter, magnetic parameter, radiation parameter on the Nusselt number and local friction coefficient, as well as temperature, velocity and angular velocity are discussed and shown graphically. It is noticed that the local skin friction and the local Nasselt number has decrement behaviors when increasing the values the Casson parameter. But the opposite happens when the mixed convection parameter λ increase. It is found that the results in this study are in good agreement with previous studies. This proves that calculations using KBM method and the chosen step size are accurate enough for this type of problems.

Generation of entropy during forced convection of heat in nanofluid stagnation-point flows over a cylinder embedded in porous media

Thermodynamics and heat transfer of an impinging nanofluid flow upon a cylinder with constant surface temperature and embedded in porous media are investigated. Numerical solutions reveal the flow velocity and temperature fields as well as the Nusselt number. These are then used to calculate the rate of entropy generation within the system by viscous and heat transfer irreversibilities. It is demonstrated that changes in the concentration of nanoparticles modify the thermal and hydrodynamic boundary layers and hence can alter the Nusselt number and entropy generation considerably. However, the shear stress on the surface of the cylinder is observed to be less affected by the variations in the concentration of nanoparticles. Further, the Reynolds number and non-uniform transpiration are shown to affect the Nusselt number and entropy generation. It is argued that the influences of Reynolds number on the boundary layer thickness can majorly modify the irreversibility and Bejan number.

Experimental investigation of wax deposition from waxy oil mixtures

Wax deposition is a common problem in oil pipelines and production systems. In this study, impact of water cut, mixing rate, chemical inhibitor, and time on wax deposition were investigated in a cold finger setup. Effect of different chemical inhibitors on wax appearance temperature (WAT) was studied using viscometry and differential scanning calorimetry techniques. Results suggested that WAT reduced with increasing inhibitor concentration, with 800 ppm being the optimum. Also, chloroform–toluene–ethylene vinyl acetate (EVA) mixture with 30, 30, and 40 wt% had the highest performance and reduced the WAT to 16.7 °C. Mixtures of toluene—EVA with acetone, p-xylene, and disulfide oil (DSO), followed next. Moreover, deposition decreased with increasing temperature difference between oil and pipe at constant cold surface temperature and increased upon increasing temperature difference at constant oil temperature. Wax deposition in two-phase system was lower than in single-phase system, but increased by increasing water cut. EVA–toluene, 2 wt% DSO, 2 wt% acetone, and 2 wt% p-xylene mixtures reduced the deposition to 23.33, 21.71, 32.14, and 12.5%, but addition of 2 wt% of EVA–DSO–acetone mixture reduced deposition to 35.74%. At similar operating conditions, flow turbulence has greater impact on reducing wax deposition, and its effect is enhanced using a proper inhibitor.

Convective Heat Transfer between Liquid Argon Flows and Heated Carbon Nanotube Arrays using Molecular Dynamics

This paper presents the molecular dynamics simulations of unconfined forced convective flow through the nanostructures at steady state condition. A better understanding of forced convective flow through the nanostructures is important because of its wide range of applications in nano-scale devices. Present work focuses on the heat transfer process of argon flow over a carbon nanotube and carbon nanotube arrays with constant surface temperature using molecular dynamics simulations. We consider two elementary configurations for the case of carbon nanotube arrays based on the unit cell structure. The simulation domain consists of fixed carbon nanotubes surrounded with the flowing argon atoms. An extensive study of momentum and thermal transport between carbon nanotube and surrounded argon atoms are analyzed from its microscopic state. The heat transfer coefficient is found in the order of 108 W/m2K. The method proposed in this paper can be an elementary step for the geometry calculation of nano-structured heat sink in the high heat flux electronic chips.

Simulative analysis of turbulent flow through a circular conduit with uniform wall temperature and constant heat flux boundary conditions

PurposeThis study aims to investigate numerically a two-dimensional fully developed mean turbulent fluid flow, and heat transfer in a circular duct is numerically investigated using FORTRAN 95 code that applies the finite difference method to solve the thermal problem for the two thermal boundary conditions, constant surface temperature, constant heat and steady, axisymmetric flow. Several important results have been drawn and discussed from thermal analysis. Finally, the numerical results of the model developed in the document have been validated in good accuracy by comparing them with some correlation results available in the specialized literature.Design/methodology/approachThe methodology of solving the thermal problem is based on the equation of energy for a fluid of constant properties while taking into consideration the hypothesis of the axisymmetric and fully developed pipe flow in steady state. The global equation and the initial and boundary conditions acting on the problem have been configured here in dimensionless form to predict the turbulent behavior of the fluid inside the tube. Thus, using Thomas' algorithm, a program in FORTRAN version 95 was developed to numerically solve the discretized form of the system of equations describing the problem.FindingsThe profiles of the solutions are provided from which the authors infer that the numerical and literature correlation agreed very well. Another result that they obtained from this study is the number of Nusselt in the thermal entrance region to which a parametric study based on Reynolds and Peclet numbers, and the longitudinal coordinate, was carried out and discussed well for the impact of the scientific contribution.Originality/valueThe novelty of the work is the application of the finite difference method programed on the FORTRAN code, as a sequential numerical method of an ODEs system, to determine the number of Nusselt in both uniform wall temperature and wall heat flux uniform.

Convective Self‐Aggregation As a Cold Pool‐Driven Critical Phenomenon

AbstractConvective self‐aggregation is when thunderstorm clouds cluster over a constant temperature surface in radiative convective equilibrium. Self‐aggregation was implicated in the Madden‐Julian Oscillation and hurricanes. Yet, numerical simulations succeed or fail at producing self‐aggregation, depending on modeling choices. Common explanations for self‐aggregation invoke radiative effects, acting to concentrate moisture in a subdomain. Interaction between cold pools, caused by rain evaporation, drives reorganization of boundary layer moisture and triggers new updrafts. We propose a simple model for aggregation by cold pool interaction, assuming a local number density ρ(r) of precipitation cells, and that interaction scales quadratically with ρ(r). Our model mimics global energy constraints by limiting further cell production when many cells are present. The phase diagram shows a continuous phase transition between a continuum and an aggregated state. Strong cold pool‐cold pool interaction gives a uniform convective phase, while weak interaction yields few and independent cells. Segregation results for intermediate interaction strength.

Heat Transfer in Air Flow Past a Bottom Channel Wall-Attached Diamond-Shaped Baffle – Using a CFD Technique

A computational analysis has been conducted to investigate turbulent flow and convective thermal transfer characteristics in a two-dimensional horizontal rectangular section channel with a hot lower wall-mounted diamond-shaped baffle. The calculations are based on the finite volume method, by means of Commercial Computational Fluid Dynamics software FLUENT, standard k-epsilon turbulence model with QUICK numerical scheme, and the SIMPLE discretization algorithm has been applied. The fluid flow and heat transfer characteristics, i.e., dynamic pressure coefficient, stream function, mean, axial, and transverse velocities, turbulent viscosity, temperature field, skin friction coefficients, local and average Nusselt numbers, and thermal enhancement factor are presented for flow Reynolds numbers based on the aeraulic diameter of the computational domain ranging from 12,000 to 32,000 at constant surface temperature condition along the upper and lower walls. Effect of the diamond configuration of the insulated baffle is studied numerically and the data obtained from this same baffle model are also compared with that of the simple flat rectangular baffle under similar operating conditions. Over the range under investigation, the improvements are found to be around 3.962 and 29.820 times higher than the smooth air channel with no baffle for heat thermal transfer and skin friction factor, respectively. The maximum TEF is around 1.292 at the highest Reynolds number value, Re = 32,000.

Experimental studies of mitigating premixed flame-excited thermoacoustic oscillations in T-shaped Combustor using an electrical heater

Abstract In this work, attenuating unsteady heat-driven thermoacoustic oscillations in a T-shaped standing-wave Rijke-type combustor is numerically and experimentally studied. For this, 2D numerical studies are conducted first on a T-shaped standing-wave combustor. In such combustor, a heater with constant surface temperature of 1100 K is confined in the bottom branch. However, a secondary heater with a controllable surface temperature is enclosed in the horizontal bifurcating branch. When the secondary heater is not actuated, large-amplitude limit cycle oscillations are successfully generated. However, as the secondary heater surface temperature is increased to 1600 K, the limit cycle oscillations are completely mitigated. To validate these findings, experimental study is then conducted on a T-shaped combustor. A premixed flame is enclosed in the bottom branch and an electrical heater is implemented to attenuate unstable combustion oscillations generated by the flame. When the electrical heater is not actuated, premixed flame-excited thermoacoustic oscillations are generated at approximately 210 Hz. However, with the heater being actuated, sound pressure level is successfully reduced from 130 dB to 85 dB. The present work opens up an alternative control approach to enable combustors being operated stably.

Parabolic Profile in Heat-Conduction Problems. 2. Semi-Bounded Space with a Time-Varying Surface Temperature

Some approaches to the optimization of the exponent n in the parabolic temperature profile for a semi-bounded space at a boundary condition with a time-varying surface temperature are considered. It is shown that solutions obtained using the scheme of T. Myers with minimization of the Langford norm are not even close to the optimum ones and that the minimization scheme based on the new error norm estimating the modulus of a temperature deviation is much more efficient. New hybrid integral schemes convenient to use are proposed. Parabolic solutions have been obtained on the basis of a number of integral relations: the temperature-momentum integrals of single and double modifications and the temperature-function integral. On the basis of these solutions in combination with known integral relations, different variations of the hybrid integral method have been developed. It was established that the highest approximation accuracy is provided by the hybrid scheme combining the temperature-momentum integrals of single and double modifications. It is shown that the hybrid integral schemes proposed are much more accurate compared to the known integral methods, including those realized using the scheme with minimization of the Langford norm. In particular, for a semi-bounded space with a surface of constant temperature, the hybrid integral method variant combining the temperature-momentum integral of single modification and the refined integral method makes it possible to obtain a simple cubic parabola representing a substantially better approximation solution compared to the temperature profile obtained on the basis of the refined integral method with optimization of the exponent n by the scheme of T. Myers.

Experimental demonstration of mitigating self-excited combustion oscillations using an electrical heater

The present work considers mitigating combustion instability occurred in a conventional Rijke-type combustor and a Y-shaped one. For this, theoretical, numerical and experimental studies are conducted to shed lights on the role of applying a heater on suppressing flame-exited oscillations. Theoretical modal analysis shows that the heater acts like an acoustic absorber. Further 2D numerical investigations are conducted on a Rijke-type combustor, in which a propane-fuelled flame and heating bands with constant surface temperatures are implemented at upstream and downstream of the combustor. Limit cycle oscillations produced by the flame are found to be significantly suppressed, if the surface temperature of the heating bands is large enough. Experimental study is then performed on a Rijke tube, in which implementing a heater leads to the unstable combustion system being successfully stabilized. Further validation of the proposed control approach is performed by implementing the electrical heater in an upper branch of a Y-shaped combustor. A premixed propane-fuelled flame is enclosed in the bottom stem. When the heater is not actuated, self-excited thermoacoustic oscillations are generated at approximately 190 Hz. However, with the heater being actuated, sound pressure level is successfully reduced from 130 dB to 80 dB. The present work opens up an alternative but practical control approach to enable combustors being operated stably.