Interacting effects of uniform flow, plane shear, and near-wall proximity on the heat and mass transfer of respiratory aerosols

Individual and interacting effects of uniform flow, plane shear, and near-wall proximity on spherical droplet heat and mass transfer have been assessed for low Reynolds number conditions beyond the creeping flow regime. Validated resolved volume simulations were used to compute heat and mass transfer surface gradients of two-dimensional axisymmetric droplets and three-dimensional spherical droplets near planar wall boundaries for conditions consistent with inhalable aerosols (5 ? d ? 300 μm) in the upper respiratory tract. Results indicate that planar shear significantly impacts droplet heat and mass transfer for shear-based Reynolds numbers greater than 1, which occur for near-wall respiratory aerosols with diameters in excess of 50 μm. Wall proximity is shown to significantly enhance heat and mass transfer due to conduction and diffusion at separation distances less than five particle diameters and for small Reynolds numbers. For the Reynolds number conditions of interest, significant non-linear effects arise due to the concurrent interaction of uniform flow and shear such that linear superposition of Sherwood or Nusselt number terms is not allowable. Based on the validated numeric simulations, multivariable Sherwood and Nusselt number correlations are provided to account for individual flow characteristics and concurrent non-linear interactions of uniform flow, planar shear, and near-wall proximity. These heat and mass transfer correlations can be applied to effectively compute condensation and evaporation rates of potentially toxic or therapeutic aerosols in the upper respiratory tract, where non-uniform flow and wall proximity are expected to significantly affect droplet transport, deposition, and vapor formation.     

Heat and mass transfer analysis of combined convection in a horizontal rectangle

In this study, the heat and mass transfer of combined free and forced convection in the horizontal rectangle is explored. The governing equations together with the boundary conditions are solved numerically by using the finite volume method. The innovative idea in this study is to appropriately modify the Semi-Implicit Method for Pressure-Linked Equations algorithm and thereby, the numerical solutions of the flow variables such as the temperature and the concentration in addition to the components of velocity and the pressure are computed. The Richardson numbers (Ri) for distinct gases and liquids are calculated for different Rayleigh numbers at low (Re = 50) and high (Re = 5000) Reynolds numbers. The dimensionless parameters, such as the Reynolds number (Re), the Prandtl number (Pr), and the Schmidt number (Sc) are appropriately chosen to calculate the Richardson numbers. Consequently, combined free and forced convection effects are analyzed. Furthermore, the heat and mass transfer aspect for distinct gases and liquids is critically examined using empirical correlations. The accuracy and the validation of these results are ensured owing to the solutions obtained from correlations being advised in this study and those are existing in the literature.     

Heat and mass transfer induced by the ignition of single gel propellant droplets

This experimental research studies the gas-phase ignition of single droplets of several gel propellant compositions based on ethyl alcohol with a gellant, liquid and fine solid combustible components. Droplets 2 mm in diameter were located on a holder and heated in a muffle furnace at a temperature ranging from 873 to 1073 K. A software and hardware system of high-speed video recording (4200 frames per second at full resolution) allowed the analysis of consistent patterns in the physical and chemical processes occurring during the induction period. For the compositions under study, we determined the threshold conditions (minimum ambient temperature of 873–943 K) required for the gel propellant ignition as well as the dependences of the ignition delay times versus air temperature. The ignition delay times range from 0.1 to 3.3 s. If the ignition does not start within this period, it will not occur after a longer heating time, since the propellant droplets evaporate completely. For the first time, using the shadow methods, we analyze the characteristics of vapor jetting during the induction period as a result of microexplosions caused by the differences in the boiling points of fuel components. The average vapor jetting speed is about 3 m/s. The size of the zones, in which the vapors slow down to zero, ranges from 6 to 8 mm. We determine the consistent patterns of changes in the diameter of the sphere-shaped gas-vapor envelope around the propellant droplet at the moment of ignition at different ambient temperatures. The higher the temperature, the higher the intensity of physical and chemical processes. This shortens the ignition delay times. At relatively high air temperatures (over 1050 K), the diameter of the flammable gas-vapor envelope around the propellant droplet at the moment of ignition is three times smaller than this value at the near-threshold ignition conditions, when the diameter of the fuel vapor envelope is about 9 mm (more than four typical initial droplet diameters). The results obtained helped us formulate a physical model of the process, which may serve as the basis for the development of a mathematical model simulating the ignition of gel propellant droplets under rapid heating. Such a mathematical model will make it possible to reliably forecast the characteristics of the process in a wide variation range of propellant properties, droplet configurations and parameters of the heating source.     

Soret and Dufour effects on free convection flow of non-Newtonian fluids along a vertical plate embedded in a porous medium with thermal radiation

This article numerically studies the combined laminar free convection flow with thermal radiation and mass transfer of non-Newtonian power-law fluids along a vertical plate within a porous medium. The solution takes the diffusion-thermo (Dufour), thermal-diffusion (Soret), thermal radiation and power-law fluid index effects into consideration. The governing boundary layer equations along with the boundary conditions are first cast into a dimensionless form by a similarity transformation and the resulting coupled differential equations are then solved by the differential quadrature method (DQM). The effects of the radiation parameter R, the power-law index n, the Dufour number D_f, and the Soret number S_r on the fluid flow, thermal and concentration fields are discussed in detail. The results indicate that when the buoyancy ratio of concentration to temperature is positive, N > 0, the local Nusselt number increases with an increase in the power-law index and the Soret number or a decrease in the radiation parameter and the Dufour number. In addition, the local Sherwood number for different values of the controlling parameters is also obtained.     

The effect of ambient pressure on the evaporation of a single droplet and a spray

The effect of ambient pressure on the evaporation of a droplet and a spray of n-heptane was investigated using a model for evaporation at high pressure. This model considered phase equilibrium using the fugacities of the liquid and gas phases for the behavior of a gas being real, and its importance in the calculation of the evaporation of a droplet or spray at high pressures was demonstrated. For the evaporation of a single droplet, the fact that the droplet's lifetime increased with pressure at a low ambient temperature, but decreased at high temperatures, was explained with pressure and the droplet's temperature determining phase equilibrium. In this study, it was also found that the evaporation of a spray can be explained in terms of multiplex dependencies of the atomization and evaporation of a single droplet. The evaporation of a spray was enhanced by increasing the ambient pressure and this effect was more dominant at higher ambient temperatures.     

Ignition and combustion of n-heptane droplets in carbon dioxide enriched environments

The ignition process and burning characteristics of fiber-supported n-heptane fuel droplets in carbon dioxide enriched and varying pressure environments have been studied under normal gravity. Measured values of droplet burning rates, flame dimensions, broad-band radiant emission, and ignition times were compared to droplets burning in standard air conditions. The burning rate constants increased with increasing carbon dioxide concentration or pressure. For 21% ambient oxygen concentration ignition was achieved for carbon dioxide concentrations up to 46% with the remaining being nitrogen. The experimental burning rates were compared to existing theoretical models. A flammability map for n-heptane burning under normal gravity as a function of carbon dioxide concentration and pressure was also developed using these results.     

Transport processes and associated irreversibilities in droplet combustion in a convective medium

A mathematical model of the combustion of a droplet surrounded by hot gas with a uniform free stream motion is made from the numerical solution of conservation equations of heat, mass and momentum in both the carrier and the droplet phases. The gas-phase chemical reaction between the fuel vapour and the oxidizer is assumed to be single-step and irreversible. The phenomenon of ignition is recognised by the sudden rise of temperature in the temperature/time histories at different locations in the carrier phase. To ascertain the process irreversibilities, the instantaneous rate of entropy production and its variation with time have been determined from the simultaneous numerical solution of the entropy conservation equations for both the gas and liquid phases. The relative influences of pertinent input parameters, namely the initial Reynolds number Re_i, the ratio of the free stream to initial temperature T_∞ and the ambient pressure on (i) the local and average Nusselt numbers, (ii) the life histories of burning fuel drops, and (iii) the entropy generation rate in the process of droplet combustion have been established.     

Heat transfer using a correlation by neural network for natural convection from vertical helical coil in oil and glycerol/water solution

A physical-empirical model is designed to describe heat transfer of helical coil in oil and glycerol/water solution. It includes an artificial neural network (ANN) model working with equations of continuity, momentum and energy in each flow. The discretized equations are coupled using an implicit step by step method. The natural convection heat transfer correlation based on ANN is developed and evaluated. This ANN considers Prandtl number, Rayleigh number, helical diameter and coils turns number as input parameters; and Nusselt number as output parameter. The best ANN model was obtained with four neurons in the hidden layer with good agreement (R > 0.98). Helical coil uses hot water for the inlet flow; heat transfer by conduction in the internal tube wall is also considered. The simulated outlet temperature is carried out and compared with the experimental database in steady-state. The numerical results for the simulations of the heat flux, for these 91 tests in steady-state, have a R ≥ 0.98 with regard to experimental results. One important outcome is that this ANN correlation is proposed to predict natural convection heat transfer coefficient from helical coil for both fluids: oil and glycerol/water solution, thus saving time and improving general system performance.     

Heat Transfer and Fluid Flow over a Single Disk in a Fluid Rotating as a Rigid Body

Laminar heat transfer problem is analyzed for a disk rotating with the angular speed ωin a co-rotating fluid (with the angular speed Ω). The fluid is swirled in accordance with a forced-vortex law, it rotates as a solid body at β= Ω/ω= const. Radial variation of the disk's surface temperature follows a power law. An exact numerical solution of the problem is obtained basing on the self-similar profiles of the local temperature of fluid, its static pressure and velocity components. Numerical computations were done at the Prandtl numbers Pr = 1(?)0.71. It is shown that with increasing βboth radial and tangential components of shear stresses decrease, and to zero value at β= 1. Nusselt number is practically constant at β= 0(?) 0.3 (and even has a point of a maximum in this region); Nu decrease noticeably for larger βvalues.     

Heat transfer around a circular cylinder in separated air flow

An experimental study has been conducted to determine the heat transfer characteristics around a circular cylinder attached to the separated flow of air shed from a fence. The fence was located vertically to the flow with a height of H = 40 mm. d/H was constant at 0.638, where d is the cylinder diameter of 25.5 mm. X/H were 0.50 and 0.775 and Y/H ranged from 0.525 to 1.50, where X and Y are, respectively, the distances between the axis of the cylinder and the front face of the fence, and the bottom wall of the test section. The Reynolds number based on the cylinder diameter and the velocity of the undisturbed flow ranged from 1.9 × 10⁴ to 6.0 × 10⁴. It was found that the maximum local Nusselt number changes drastically in the vicinity of Y/H = 1.0–1.11 and that the maximum mean Nusselt number occurs in the neighborhood of Y/H = 1.24–1.43 for X/H = 0.50 and 1.3–1.4 for X/H = 0.775. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 211–226, 1999     

A parametric study of burning of multicomponent droplets in a dilute spray

The model for sphericosymmetric thin‐flame combustion of a multicomponent fuel droplet in a dilute spray using a unit cell approach, developed in the companion paper, has been used for studying the interaction effect between droplets. The effects of droplet spacing, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.     

Heat transfer and friction factor correlations for a duct having dimple-shape artificial roughness for solar air heaters

The heat transfer coefficient between the absorber plate and air can be considerably increased by using artificial roughness on the underside of the absorber plate of a solar air heater duct. Under the present work, an experimental study has been carried out to investigate the effect of roughness and operating parameters on heat transfer and friction factor in a roughened duct provided with dimple-shape roughness geometry. The investigation has covered the range of Reynolds number (Re) from 2000 to 12,000, relative roughness height (e/D) from 0.018 to 0.037 and relative pitch (p/e) from 8 to 12. Based on the experimental data, values of Nusselt number (Nu) and friction factor (f_r) have been determined for different values of roughness and operating parameters. In order to determine the enhancement in heat transfer and increment in friction factor values of Nusselt number and friction factor have been compared with those of smooth duct under similar flow conditions. Correlations for Nusselt number and friction factor have been developed for solar air heater duct provided such artificial roughness geometry.     

High-pressure combustion of submillimeter-sized nonane droplets in a low convection environment

An experimental study of droplet combustion of nonane (C₉H₂₀) at elevated pressures burning in air is reported using low gravity and small droplets to promote spherical gas-phase symmetry at pressures up to 30 atm (absolute). The initial droplet diameters range from 0.57 to 0.63 mm and they were ignited by two electrically heated hot wires positioned horizontally on opposite sides of the droplet. The droplet and flame characteristics were recorded by a 16-mm high-speed movie and a high-resolution video camera, respectively. A photodiode is used to measure broadband gray-body emission from the droplet flames and to track its dependence on pressure. Increasing the pressure significantly influences the ability to make quantitative measurements of droplet, soot cloud, and luminous zone diameters. At pressures as low as 2 atm, soot aggregates surrounding the droplet show significant coagulation and agglomeration and at higher pressures the soot cloud completely obscures the droplet, with the result being that the droplet could not be measured. Above 10 atm radiant emissions from hot soot particles are extensive and the resulting flame luminosity further obscures the droplet. Photographs of the luminous zone in subcritical pressures show qualitatively that increasing pressure produces more soot, and the mean photodiode voltage output increases monotonically with pressure. The maximum flame and soot shell diameters shift to later times as pressure increases and the soot shell is located closer to the flame at higher pressure. The soot shell and flame diameter data are correlated by a functional relationship of reduced pressure derived from scaling the drag and thermophoretic forces on aggregates that consolidates all of the data onto a single curve.     

Heat transfer analysis of plain and dimpled tubes with different spacings

Heat transfer enhancement by modifying the surface of tubes is commonly practiced throughout the world. Grooves, dimples, flutes or corrugations are placed inside and outside the surface of tubes and channels for enhancement. In this article, a novel method for heat transfer enhancement by varying the spacing between the tubes is reported. A comparison is made between the heat transfer performance of plain tubes and dimpled tubes at different spacings. For analysis, an experimental setup is fabricated and assembled. The flow is externally forced laminar flow of air over a hot tube maintained at constant temperature. Four different velocities of air 0.4, 0.6, 0.8, and 1.0 m/s are considered in this study. Tube surface temperature, heat transfer coefficient, heat transfer rate and Nusselt number are the parameters studied to analyze the thermal behavior of tubes at different spacings of 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 cm. From the experimental investigation it is found that, apart from heat transfer enhancement by providing dimples on the tube surfaces, there is an optimal spacing between the tubes after which no further improvement is obtained. In this study, 3.0 cm is found to be the optimal spacing for both plain and dimpled tubes. However, the percentage value of heat transfer enhancement is greater with optimal spacing and for all velocities of air in dimpled tubes.     

A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Disquisition of thermophoresis and chemical reaction sketch in viscous stream over a wedge

A computative exploration is engraved on synchronized heat and mass transmission in a stream across a wedge. The problem has been executed for MHD incompressible viscous fluid, which annexes the impact of thermophoresis. The sequels of chemical reactions and energy sources have also been encountered. The resulting equations are transmuted as nondimensional forms and a numerical method is executed to solve the same. The numerical results for velocity, temperature, and concentration distributions are illustrated in the form of graphs for various dimensionless parameters affecting the problem. Numerical data are computed for skin friction coefficient, Nusselt number, and Sherwood number for different parameters. The results have been depicted for possible cases by means of pictorial visualizations. The present study has importance in various sectors of industrial and engineering applications.     

Numerical prediction of fluid flow and heat transfer in a wavy pipe

IntroductionA pipe with periodically converging-divergingcross-section is one Of the sevens devices employed forenhancing the heat and mass tusfer efficiency. Thenuid flow, to the now passages with a periodicallyvaling cross-section, attains a folly develOPed acmethat differs fundamentally from that for a convelltionalconstant-area flow channel. In the periodically vwigcross-seCtions, the ac developed VelM field repeatsitSelf at cormsponding edal locations in successivecycles. The change of…     

Effect of heat and mass transfer on non-Newtonian flow – Links to atherosclerosis

The present investigation deals with a mathematical model representing the dynamic response of heat and mass transfer to blood streaming through the arteries under stenotic condition. The blood is treated to be a generalized Newtonian fluid and the arterial wall is considered to be rigid having differently shaped stenoses in its lumen arising from various types of abnormal growth or plaque formation. The nonlinear unsteady pulsatile flow phenomenon unaffected by the concentration-field of the macromolecules is governed by the Navier–Stokes equations together with the equation of continuity while those of the heat and the mass transfers are controlled by the heat conduction and the convection–diffusion equations, respectively. The governing equations of motion accompanied by the appropriate choice of the boundary conditions are solved numerically by Marker and Cell (MAC) method in order to compute the physiologically significant quantities with desired degree of accuracy. The necessary checking for numerical stability has been incorporated in the algorithm for better precision of the results computed. The quantitative analysis carried out finally includes the respective profiles of the flow-field, the temperature and the mass concentration along with their individual distributions over the entire arterial segment as well. The key factors like the wall shear stress and the Sherwood number are also examined for further qualitative insight into the heat flow and mass transport phenomena through arterial stenosis. The present results show quite consistency with several existing results in the literature which substantiate sufficiently to validate the applicability of the model under consideration.     

Heat and mass transfer in evaporating droplets in interaction: Influence of the fuel

The prediction of heat and mass transfer in fuel sprays is a key issue in the design of combustors where the fuel is injected in a liquid form. The development and validation of new physical models requires reliable experimental data. This paper reports on an experimental study to characterize the Nusselt and Sherwood numbers of monodisperse droplets made of fuels having different volatilities and evaporating into flowing hot air. Simultaneous measurements of the droplet size and mean temperature allowed evaluating the heat fluxes that take part in the evaporation. The experimental Nusselt and Sherwood numbers are then compared to the case of an isolated droplet. It appears that these numbers are particularly dependent on the interactions between the droplets in a way that depends on the fuel nature.