International Communications in Heat and Mass Transfer (v.32, #1-2)

The purpose of the present study was to investigate whether or not a physical water treatment (PWT) reduced the surface tension of hard water. Two different PWT devices were used: a permanent magnet—Drexel University (PMDU) and a solenoid coil electronic device (SCED). The effects of the treatment number of the PWT on the surface tension were studied. Two separate experiments were conducted: one was the measurement of surface tension, and the other was a flow-visualization of dye behavior in water samples. As the number of treatments of the PWT increased, the surface tension of the sample water decreased, a phenomenon that was consistent with the results in the dye flow-visualization experiment.
Keywords: Physical water treatment; Surface tension; Dye-flow visualization;

In this paper, a composite parallel plate channel whose central part is occupied by a clear fluid and whose peripheral part is occupied by a fluid-saturated porous medium is considered. The modeling is based on the assumption that the flow in the clear fluid region is turbulent while in the porous region the flow remains laminar. The turbulent and laminar flow solutions are matched at the porous/fluid interface, which is assumed rough. Two different models are utilized for calculating turbulent viscosity in the clear fluid region, the algebraic Cebeci–Smith model and a kε model. Numerical results obtained utilizing both models are compared and analyzed in detail.
Keywords: Turbulence; Porous media; kε model; Rough interface;

Computational analysis of the feasibility of a micro-pulsejet by Qun Wan; W.L. Roberts; A.V. Kuznetsov (19-26).
This paper investigates the feasibility of a 2-cm micro-pulsejet by numerically simulating the inviscid gas dynamic phenomena within the exhaust tube and comparing them with those for a pulsejet on the order of 50 cm in length. After initial combustion, the pressure wave propagates towards the exit and reflects back as a rarefaction wave, which generates a minimum pressure in the combustion chamber. This low pressure must be sufficient to open the reed valves to allow fresh reactants to enter. It is shown that for both large and micro-pulsejets, the minimum pressure is low enough. The characteristic operating frequency is found to be approximately inversely proportional to the pulsejet length. Estimation of the boundary layer thickness in the pulsejet shows that viscosity plays a very significant role in the micro-pulsejet and cannot be neglected.
Keywords: Pulsejet; Micro-size engine; Pulse combustion; Numerical modeling;

Pool boiling of saturated FC-72 on nano-porous surface by Srinivas Vemuri; Kwang J. Kim (27-31).
Pool boiling heat transfer from nano-porous surface immersed in a saturated FC-72 dielectric fluid has been experimentally studied at atmospheric pressure (101 kPa). The data obtained from nano-porous surface (Anodisc 25) of thickness about 70 μm made from aluminum oxide (Al2O3) obtained from Whatman, were compared to that of a plain surface (aluminum) of thickness about 105 μm. From the experimental data obtained it was evident that there is a reduction of about 30% in the incipient superheat for the applied power for nano-porous surface over plain surface. SEM photographs of the nano-porous coating were taken for determining the size of the pores.
Keywords: Pool boiling; Nano-porous surface; Enhanced surface;

Stability of free convection is examined in a vertical cavity with a fixed lateral temperature difference on the walls, which is also heated from the bottom by a constant heat flux causing vertical stratification of the base flow. Weakly nonlinear stability equations are derived, under the assumption of fully developed flow conditions. Critical Rayleigh numbers and Landau coefficients are determined in terms of the Prandtl number and stratification parameter. It is found that critical disturbances are always two-dimensional, for stationary and oscillating instabilities as well. Alternatives are considered for the calculation of the Landau coefficient. Analysis reveals that unstationary instability is the preferred mode at the codimension point.
Keywords: Vertical stability; Stratification; Lateral temperature gradient;

The conjugate gradient method with adjoint equations is applied to the natural convection problem in a porous medium for the determination of an unknown heat source which is dependent on a solute concentration generation rate. The direct, sensitivity and adjoint equations are given for a Boussinesq fluid, over an arbitrary domain in two dimensions. Solutions by control volumes are presented for a square enclosure under known temperature and concentration boundary conditions, assuming a source term proportional to the vertical average generation rate of a solute concentration governed by a Monod model. Reasonably accurate solutions are obtained at least up to Ra=105.
Keywords: Inverse convection; Porous medium; Biochemical heat source;

Stability of gravitactic micro-organisms in a fluid-saturated porous medium by T. Nguyen-Quang; A. Bahloul; T.H. Nguyen (54-63).
A study is made of the stability of motile suspensions in a horizontal porous layer. The micro-organisms are assumed to have a gravitactic behaviour, swimming randomly, but on the average upward with a constant velocity V c. The resulting equilibrium state is potentially unstable as a denser layer of micro-organisms is formed on top of a lighter one. The basic mechanism is analogous to that of Bénard convection in a fluid layer heated from below. The fluid flow is governed by the Darcy equation while the conservation of micro-organisms is described by a diffusion–convection equation similar to the conservation of energy. The problem depends on two parameters, namely the Rayleigh number and the swimming velocity V c. The present paper is focused on the stability of the equilibrium diffusive state. The stability diagram and the critical conditions for the onset of convection are obtained for a wide range of swimming velocity. It is found that if V c is very small, the critical wavenumber is zero, corresponding to a very long cell (parallel flow), but as V c is increased, the critical wavenumber also increases, corresponding to narrower flow patterns.
Keywords: Bioconvention; Gravitactic micro-organisms; Porous medium; Stability;

Bioconvection of gravitactic microorganisms in a fluid layer by A. Bahloul; T. Nguyen-Quang; T.H. Nguyen (64-71).
A study is made of the spontaneous pattern formation of gravitactic microorganisms in a horizontal fluid layer. The linear stability theory is used to determine the onset of convection in terms of the Rayleigh number and the swimming velocity. It is found that the onset of convection in a gravitactic suspension may be very different from that of Bénard cells under the well-known fixed-flux heating condition.
Keywords: Bioconvection; Stability; Fluid medium; Gravitactic micro-organism;

This article applies Reichardt's velocity profile to turbulent convection analysis. In contrast to a conventional law-of-the-wall formulation with three regions, a single profile represents the entire region from the viscous sublayer to the fully turbulent region. Special shape functions are developed for this profile in a hybrid finite element/volume formulation, together with dissipation rate boundary conditions, which are consistent with the velocity profile modeling. Applications to turbulent channel flow are successfully predicted with a kɛ turbulence model.
Keywords: Reichardt's Law; Turbulent channel flow; CVFEM;

Moisture transfer models for slabs drying by E. Kavak Akpinar; I. Dincer (80-93).
This paper deals with an experimental and theoretical investigation of drying of moist slabs. Experimental part includes the measurement of the moisture content distributions of eggplant slices with 5 mm thickness and 35 mm diameter during drying at the temperatures of 55 °C, 65 °C, and 75 °C and the velocities of 1.0 and 1.5 m/s, respectively. Four drying models are used to determine drying process parameters (e.g., drying coefficient, lag factor, and half-drying time) and moisture transfer parameters (e.g., moisture diffusivity and moisture transfer coefficient), and to calculate moisture content distributions. The calculated values are then compared with the experimental moisture data during the drying of eggplant slices at different drying air temperatures and flow velocities. An excellent agreement is obtained between the calculations and experimental measurements for the cases considered. Also, the experimental drying times are determined and compared with the ones obtained through four different drying models. The results show that all four models are capable of estimating the drying parameters and moisture content distributions. The experimental drying data and model findings are expected to be useful to drying industry.
Keywords: Drying; Moisture transfer; Moisture diffusion; Models; Slab;

The problem of laminar natural convection heat transfer in a square cavity with an adiabatic arc shaped baffle is analyzed in this paper. As boundary conditions of the cavity, two vertical opposite walls are kept at constant but different temperatures and the remaining two walls are kept thermally insulated. Results are presented for a range of Rayleigh numbers, arc lengths of the baffle, and shape parameters of the baffle. Finally, parametric results are presented in terms of isothermal lines and streamlines. It is identified that flow and thermal fields are modified by the blockage effect of the baffle. The degree of flow modification due to blockage is enhanced by increasing the shape parameter of the baffle.
Keywords: Baffle; Energy transfer; Finite volume method; Natural convection; Nusselt number;

For hybrid media, involving both a porous substrate and an unobstructed flow region, difficulties arise due to the proper mathematical treatment given at the macroscopic interface. The literature proposes a jump condition in which shear stresses on both sides of the interface are not of the same value. This paper presents numerical solutions for such hybrid medium, considering here a channel partially filled with a porous layer through which an incompressible fluid flows in turbulent regime. Here, diffusion fluxes of both momentum and turbulent kinetic energy across the interface present a discontinuity in their values, which is based on a certain jump coefficient. Effects of such parameter on mean and turbulence fields around the interface region are numerically investigated. Results indicate that depending on the value of the stress jump parameter, a substantially different structure for the turbulent field is obtained.
Keywords: Interface; Jump conditions; Turbulence; Porous media; Environmental flows;

Critical size of aerosol particles in the resonance radiation field by V.V. Levdansky; J. Smolik; P. Moravec (116-122).
The problem of the critical (equilibrium) size of a small aerosol particle (drop) and the critical thickness of a thin liquid film on the particle with account for the resonance radiation effect is considered. Joint influence of heat and resonance effects is discussed.
Keywords: Aerosol particle; Condensation; Critical size;

On heat conduction problem in a semi-infinite periodically laminated layer by Roman Kulchytsky-Zhyhailo; Stanisław J. Matysiak (123-132).
The paper deals with the heat conduction problem of a semi-infinite periodically stratified layer heated by a constant heat flux directed according to the layering, normal to the boundary being a cross-section of the composite components. The free heat exchange with surroundings is considered on the remaining parts of the boundary. The body is assumed to be composed of n periodically repeated two-layered, perfect bonding lamine. The problem is solved on two ways: (1) directly as heat conduction problem or (2) by using the homogenized model with microlocal parameters [R. Kulchytsky-Zhyhailo, S.J. Matysiak, On some heat conduction problem in a periodically two-layered body. Comparative results, Int. Commun. Heat Mass Transf., in press]. The obtained results are compared and presented in the form of figures.
Keywords: Laminated composite; Temperature; Heat flux; Semi-infinite layer; Homogenized model;

Radiative transfer calculations for the hemispherical transmittance through a packed bed of opaque spheres are performed based on a correlated radiative property model previously proposed by one of the authors and obtained results are favorably compared with available results of direct Monte Carlo simulations. It is also shown that the scaling factor S r to the extinction coefficient of packed spheres empirically determined by Singh and Kaviany closely resembles the scaled extinction enhancement factor (2γ 2−1) theoretically derived by Kamiuto, except in a low porosity region.
Keywords: Correlated radiative transfer; Packed bed; Opaque sphere; Hemispherical transmittance;

The heat transfer characteristics and performance of the double-pass flat plate solar air heater with and without porous media are studied numerically. The mathematical models described the heat transfer characteristics of the double-pass flat plate solar air heater are derived from the energy conservation equations. The implicit method of finite-difference scheme is employed to solve these models. The effect of the thermal conductivity of the porous media on the heat transfer characteristics and performance is considered. The results obtained from the model are validated by comparison with experimental data of previous researchers. There is reasonable agreement between the present model and experiment.
Keywords: Porous media; Performance; Double-pass solar air heater;

A total of 23 cross-flow heat exchangers having crimped spiral configurations is studied. The effect of tube diameter, fin spacing, transverse tube pitch, and tube arrangements are examined. For the inline arrangement, the pressure drop increases with the rise of tube diameter but the associated heat transfer coefficient decreases with it. The increase of fin height also gives rise to considerable increase of pressure drop and decrease of heat transfer coefficients for the inline arrangement. However, for the staggered arrangement, the effect of the fin height on the pressure drop is much smaller than that of the inline arrangement due to the major contribution to the total pressure drops from the blockage of the airflow from staggered arrangement. Effect of the fin spacing on the air side performance is strongly related to the transverse tube pitch for both inline and staggered arrangements. Correlations of the present crimped spiral fins in both staggered and inline arrangements are developed. The proposed correlations give fairly good predictive ability against the present test data.
Keywords: Heat exchanger; Performance testing; Crimped spiral fins;

Experiments are performed to study the single-phase transient heat transfer at different protrusion heights of the electronic chips during an accidental stoppage of coolant flow due to loss of pumping power. Water is the coolant media and the flow covers the wide range of laminar flow regime with Reynolds number, based on heat source length, from 800 to 2625 and the heat flux ranging from 1 W/cm2 to 7 W/cm2. The general impacts of heat source protrusions (B=1, 2 mm) on heat transfer behavior of four chips are investigated by comparing the results obtained from flush-mounted (B=0) heat sources. Finally the correlation equations are obtained by modifying the correlations of flush-mounted heater, employing the chips' protrusion height (B) in the dimensionless (B/ℓ) form.
Keywords: Discrete heat source; Electronic cooling; Natural convection; Transient heat transfer;

Condensation of steam in silicon microchannels by Yongping Chen; Ping Cheng (175-183).
A visualization study was performed on condensation of steam in microchannels etched in a 〈100〉 silicon wafer that was bonded by a thin Pyrex glass plate from the top. The microchannels had a trapezoidal cross section with a hydraulic diameter of 75 μm. Saturated steam flowed through these parallel microchannels, whose walls were cooled by natural convection of air at room temperature. The absolute pressure of saturated steam at the inlet ranged from 127.5 kPa to 225.5 kPa, and the outlet was at atmospheric pressure at approximately 101.3 kPa with the outlet temperature of the condensate ranging from 42.8 °C to 90 °C. Stable droplet condensation was observed near the inlet of the microchannel. When the condensation process progressed along the microchannels, droplets accumulated on the wall. As the vapor core entrained and pushed the droplets, it became an intermittent flow of vapor and condensate at downstream of the microchannels. The traditional annual flow, wavy flow and dispersed flow observed during condensation in macrochannels were not observed in the microchannels. Based on a modified classical droplet condensation theory, it is predicted that the droplet condensation heat flux increases as the diameter of the microchannel is decreased. It is also predicted that the droplet condensation heat flux of saturated steam at 225.5 kPa can reach as high as 1200 W/cm2 at ΔT=10 °C in a microchannel having a hydraulic diameter of 75 μm.
Keywords: Condensation; Microchannel; Phase change;

The heat transfer analysis of considering heat-generating components with different locations of two side-wall fans in a three-dimensional desktop computer was investigated in this paper. Herein, the well-known computational fluid dynamics (CFD) code of PHOENICS was employed to simulate the dissipative heat transfer in a ventilated enclosure. The SIMPLEST algorithm with the hybrid scheme was utilized to simulate these flows. The parameters are focused on the inlet Reynolds number and the locations of two fans on one of the side-wall boards. The calculating results show that the heat transfer efficiency of mode 4 is better than the other three modes due to the directly dissipative heat by forcing fans right on the vicinity of the high heat-generating components. The present findings not only set up a numerical heat transfer analysis of desktop computer but also provide a basis for further simulation of the associated heat transfer for more complicated situations.
Keywords: Desktop computer; Computational fluid dynamics; Heat transfer;

The qualitative effect of a non-uniform basic temperature gradient on the stationary and oscillatory stability analyses of the onset of Bénard-Marangoni convective in a horizontal fluid layer is investigated numerically using the fourth order Runger-Kutta-Gill's method coupled with the iterative Broyden's method. The effects of crispation at a deformable upper free surface and the conductive effect of non-steady conditions within the fluid layer on onset are discussed. Results show that the effect of crispation (i.e., Crispation number C) is a clearly destabilizing factor, but the conductive effect (i.e., a i * of non-steady conditions within the fluid layer does play a stabilizing state. The system becomes more stable at larger values of the Prandtl number Pr, the Biot number B i and the Bond number B o . We obtain a set of characteristic curves in the (M C, R)-plane that satisfy the linear relation on the onset of the stationary and oscillatory Bénard-Marangoni convective instability.
Keywords: Crispation; Stationary; Oscillatory; Convective instability;

This work uses the integral method to study the heat and mass transfer by natural convection from vertical plates with variable wall temperature and concentration in porous media saturated with an electrically conducting fluid in the presence of a transverse magnetic field. The surface temperature and concentration are assumed to vary as a power of the axial coordinate measured from the leading edge of the plate. The approximate solutions are found to be in reasonable agreement with the similarity solutions. Results are plotted for the local Nusselt number, the local Sherwood number, and the reciprocal of the ratio of the thermal boundary-layer thickness to the concentration boundary-layer thickness. Increasing the power-law exponents tends to increase the local Nusselt number and the local Sherwood number. Increasing the magnetic parameter decreases the local Nusselt number and the local Sherwood number. Moreover, the ratio of the thermal boundary-layer thickness to the concentration boundary-layer thickness increases with the Lewis number, and it also increases with the buoyancy ratio when the Lewis number is not equal to one.
Keywords: Hydromagnetic natural convection heat; Heat and mass transfer; Power-law variation;

The present paper analyses the entropy generation of the fully developed laminar convection in a helical coil with constant wall heat flux and presents the optimal design based on the minimum entropy generation principal. The important design parameters, including Reynolds number (Re), coil-to-tube radius ratio (δ) and nondimensional coil pitch (λ) are varied to investigate their influences on the entropy generation. The results presented in this paper cover Re range of 100–10,000, δ and λ range from 0.01 to 0.3. Compared with Re and δ, the coil pitch λ is found to have minor influence on the entropy generation. For a demonstrated case, the minimum entropy generation occurs in the range bounded by Re from 2271 to 4277 and δ from 0.17 to 0.3, within which the irreversibility of the system is lowest and the system performance would be optimum. The details show that there is an optimal Re for a helical coil with a fixed δ; meanwhile for a helical coil flow with a specified Re, the smaller δ should be selected when the Re is larger than 5000, and the larger δ should be selected when the Re is less than 5000. These results provide worthwhile information for heat exchanger designers to find the optimal helical coil design from the viewpoint of the thermodynamic second law.
Keywords: Entropy generation; Helical coil; Irreversibility; Thermodynamic second law;

Fluid flow and heat transfer in fluidized bed vertical shell and tube type heat exchanger by S.W. Ahn; S.T. Bae; B.C. Lee; W.C. Kim; M.W. Bae (224-232).
Measurements were made on the effects of circulating solid particles on the characteristics of fluid flow and heat transfer in the fluidized bed vertical shell and tube type heat exchanger with counterflow. The present work showed that the flow velocity range for collision of particles to the tube wall was higher with heavier density solid particles, and the increase in heat transfer was in the order of sand, copper, steel, aluminum, and glass.
Keywords: Fluidized bed type; Heat exchanger; Solid particles; Heat transfer coefficient; Counterflow;

A programme of condensation experiments was carried out at atmospheric pressure using a water-cooled flat plate varying the air concentrations, the plate inclination and its orientation (upward or downward facing). Rates of heat transfer have been measured on a single face of the condensing plate suspended in a cylindrical test section as steam and mixtures of steam and air flowed over it. The rate of heat transfer decreased as the angle of the plate to the horizontal was reduced and as the concentration of air was increased. A notable observation was that comparison of results for the upward and downward facing cases showed that the heat transfer rates with pure steam are higher for an upward facing plate than for a downward facing one. However, with air present in the steam, this trend is reversed. The effects of plate orientation, mixture flows and buoyancy are discussed.
Keywords: Condensation; Film-wise condensation; Inclined plate; Plate orientation; Stem; Air;

Effect of a longitudinally positioned solenoid coil on electronic descaling by C.H. Sohn; C.S. Kim; S.Y. Moon; Y.I. Cho (240-247).
The present study introduces a new method in electronic descaling (ED) technology. In the proposed method, the ED apparatus consisted of a longitudinally positioned solenoid coil installed on an outer surface of a pipe. When electric current flows through the electric coils a magnetic field is generated whose direction is perpendicular to that of the flow direction. Experiments were performed at various Reynolds numbers. In order to monitor fouling at the heat transfer test section, the pressure drop across the test section and the overall heat transfer coefficient were measured as a function of time. The present study demonstrated an enhanced descaling effect of the longitudinally positioned solenoid coil while effectively inhibiting a formation of scales at slow flow conditions.
Keywords: Electronic descaling; Solenoid coil;

Convection heat transfer from an array of discrete heat sources inside a rectangular channel has been investigated experimentally for air. The lower surface of the channel was equipped with 8×4 flush-mounted heat sources subjected to uniform heat flux; the sidewalls and the upper wall were insulated and adiabatic. The experimental parametric study was made for an aspect ratio of AR=2, Reynolds numbers 864≤Re D h ≤7955, and modified Grashof numbers Gr*=1.72×108 to 2.76×109. From the experimental measurements, surface temperature distributions of the discrete heat sources were obtained and effects of Reynolds and Grashof numbers on these temperatures were investigated. Furthermore, Nusselt number distributions were calculated for different Reynolds and Grashof numbers. Results show that surface temperatures increase with increasing Grashof number and decrease with increasing Reynolds number. However, with the increase in the buoyancy affected secondary flow and the onset of instability, temperatures level off and even drop as a result of heat transfer enhancement. This outcome can also be observed from the variation of the row-averaged Nusselt number showing an increase towards the exit.
Keywords: Convection heat transfer; Discrete heat sources;

The non-Darcian effect on forced convection heat transfer over a flat plate in a porous medium is examined. The fluid viscosity is assumed to vary as an inverse linear function of temperature. The effects of inertia forces and the distance from the leading edge of the plate on the velocity and temperature fields as well as on the skin friction and heat transfer coefficients in the boundary layer over a semi-infinite plate are studied. The nonlinear boundary layer equations, governing the problem under consideration, are solved numerically by applying an efficient numerical technique based on the Keller box method. The velocity profiles, temperature profiles and the skin friction components on the plate are computed and discussed in detail numerically for various values of the variable viscosity parameter, the modified Reynolds number, the stream wise coordinate and the Prandtl number.
Keywords: Non-Darcian; Forced convection; Heat transfer; Porous medium;

Entropy generation for semi-cylindrical ducts is obtained analytically for laminar flow and subjected to constant wall heat flux boundary conditions. Affecting parameters such as heat flux rate, Reynolds number and cross sectional are studied for entropy generation. It is concluded that cross-sectional area and wall heat flux have considerable effect on entropy generation. For the increasing value of these parameters, both entropy generation and pumping power ratio are increased at fixed Reynolds number.
Keywords: Second law analysis; Laminar flow; Semi-circular duct;

The paper presents results of an experimental study of heat transfer and friction in rectangular ducts with baffles (solid or perforated) attached to one of the broad walls. The duct has width-to-height ratio of 7.77; the baffle pitch-to-height ratio is 29; the baffle height-to-duct height ratio is 0.495. The Reynolds number of the study ranges from 2850 to 11500. The baffled wall of the duct is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. Over the range of the study, the Nusselt number for the solid baffles is 73.7–82.7% higher than that for the smooth duct, while for the perforated baffles, it ranges from 60.6–62.9% to 45.0–49.7%; decreasing with the increase in the open area ratio of these baffles from 18.4% to 46.8%. The friction factor for the solid baffles is found to be 9.6–11.1 times of the smooth duct, which decreased significantly for the perforated baffles with the increase in the open area ratio; it is only 2.3–3.0 times for the perforated baffles with open area ratio of 46.8%. Performance comparison with the smooth duct at equal pumping power shows that the baffles with the highest open area ratio give the best performance.
Keywords: Heat transfer enhancement; Rectangular duct with baffles; Thermo-hydraulic performance;