Forced-convection freezing heat transfer characteristics in a return bend with a rectangular cross section

Experiments have been performed to investigate the freezing heat transfer characteristics in a return bend with a rectangular cross section. The experiments were carried out for two kinds of duct heights of 30 and 50 mm under the fixed size of 300 mm in duct width and 159 mm in curvature radius of convex wall. Both the convex and concave walls of a return bend were kept less than the freezing temperature of water. It was found that the freezing characteristics on the convex wall are markedly different from those on the concave wall of a return bend, and that the cooling temperature ratio is one of the most important parameters on the forced-convection freezing heat transfer in a return bend.     

Velocity characteristics of the flow around a square cross section cylinder placed near a channel wall

Laser-Doppler measurements are reported of the flow around a square cross section cylinder placed at various heights (Y ₀) above a plane channel wall for a Reynolds number Re _H = 1.36 × 10⁴.The thickness of the turbulent boundary layer on the channel wall at the obstacle position, but with it removed from the water tunnel, was equal to 0.8 H, being H the square obstacle height and the free stream turbulence intensity was 6%. The periodic character of the flow in the near wake was characterized by measurements of turbulence spectra in the range 0 Y ₀/H 3.3 and the results revealed that regular vortex shedding was suppressed for a gap height less than 0.35 H. Detailed results of time averaged mean flow properties, turbulence intensities and Reynolds stresses revealed the structural differences of the near wakes with and without vortex shedding for Y _O = 0.5 and Y _O = 0.25 respectively.List of symbols R _e Reynolds number Re = U ₀ H/v - H Square obstacle height - Y Normal coordinate - Y ₀ Gap distance between the plane wall and obstacle face - C _f Skin friction coefficient - k Turbulent kinetic energy - u Friction velocity - Turbulent boundary layer thickness - y ⁺ Non-dimensional log law coordinate - X _r Separated flow length behind the obstacle - U Mean axial velocity - V Mean radial velocity - u² Axial turbulent stress - ² Normal turbulent stress - u Turbulent shear stress - U ₀ Mean bulk velocity - E Power spectrum energy - f Predominant frequency - h Distance between inner shear layers behind the obstacle - s Distance from the wall to the shear layer behind the obstacle - C _D Drag coefficient     

Sizing problem for a cross flow heat exchanger with wing-type vortex generator

In the present study, sizing of a single pass cross flow heat exchanger with unmixed fluid streams has been investigated. The heat exchanger is a cross flow heat exchanger. It has overall dimensions of 20 × 20 × 20 cm. Two the most common heat exchanger design problems are the rating and sizing problem. Sizing problems deal with designing an exchanger and determining its physical size to meet the specified heat duty, pressure drops and other considerations. It means the determination of the exchanger construction type, flow arrangement, heat transfer surface geometries and materials, and the physical sizes of an exchanger to meet specified heat transfer and pressure drop. In this study, the physical size (length, width, height, mass flow rates of both fluids and surface areas on each side of the exchanger) are determined. Inputs to the sizing problem are surface geometries, fluid mass flow rates, inlet and outlet fluid temperatures and pressure drop on each side. Dimensions of L _a , L _b , and L _c for the selected surfaces were investigated such that the design meets the heat duty and pressure drops on both sides exactly.     

Heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a heat exchanger

This paper describes heat and mass transfer characteristics of organic sorbent coated on heat transfer surface of a fin-tube heat exchanger. The experiments in which the moist air was passed into the heat exchanger coated with sorption material were conducted under various conditions of air flow rate (0.5–1.0 m/s) and the temperature of brine (14–20°C) that was the heat transfer fluid to cool the air flow in the dehumidifying process. It is found that the sorption rate of vapor is affected by the air flow rate and the brine temperature. Meanwhile, the attempt of clarifying the sorption mechanism is also conducted. Finally the average mass transfer coefficient of the organic sorbent coated on heat transfer surface of a fin-tube heat exchanger is non-dimensionalzed as a function of Reynolds number and non-dimensional temperature, and it is found that the effect of non-dimensional temperature on them is larger than Reynolds number .     

Heat transfer characteristics of a new helically coiled crimped spiral finned tube heat exchanger

In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil. Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate. The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing inlet water temperature.     

Heat transfer and thermal performance characteristics of heat exchanger tube fitted with perforated twisted-tapes

Twisted tape insert was applied as a swirling flow generator for the passive heat transfer enhancement in the present work. The influences of the perforated twisted tapes (PTs) on the heat transfer, pressure loss and thermal performance characteristics were investigated experimentally. The experiments were performed under uniform wall heat flux condition by using PTs with y/W?=?3, 4 and 5, d/W?=?0.11, 0.14 and 0.17 and s/W?=?0.4, 0.6 and 0.8 where y is a twist length, d is a perforation hole diameter, s is a spacing between holes (pitch) and W is a tape width. The experimental results reveal that Nusselt number increased with decreasing s/W and y/W and increasing d/W. For the present range, the maximum heat transfer was obtained by utilizing the tape with s/W?=?0.4, d/W?=?0.17 and y/W?=?3, which is higher than those obtained from the plain tube with and without typical twisted tape by around 27.4 and 86.7%, respectively. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance are also proposed in the present paper.     

Nonlinear diffusion-induced stresses in a long bar of square cross section

This paper is to investigate the nonlinear effect of the self-induced electric field on the diffusion-induced stresses in a long bar. We first approximate the nonlinear concentration-dependent diffusivity as a series of third-degree polynomials by the least-squares curve-fitting techniques, and then calculate the distributions of concentration by the Galerkin method. Afterwards, the diffusion-induced stresses inside the bar are determined analytically by introducing the Goodier displacement potential and Airy stress function. It is found that the nonlinear self-induced electric fields can depress both the concentration gradient and the maximum diffusion-induced stresses apparently, and these effects are more significant at short times than at long times.     

Heat propagation in a nonuniform rod of variable cross section

An integral formula is used to average a coupled problem of thermoelasticity for a nonuniform rod of variable cross section. Effective characteristics are found. It is shown that, in addition to the expected effective coefficients, there appear five independent coefficients characterizing the temperature change rate effect on the stresses in the rod, on the longitudinal heat flux, and on the entropy distribution along the length of the rod. A feature of these new coefficients is that they become equal to zero in the case of a uniform rod. The homogenization of the thermoelasticity equations for nonuniform rods allows one to propose a new theory of heat conduction in rods. This new theory differs from the classical one by the fact that some new terms are added to the Duhamel–Neumann law, to the Fourier heat conduction law, and to the entropy expression. These new terms are proportional to the temperature change rate with time. It is also shown that, in the new theory of heat conduction, the propagation velocity of harmonic heat perturbations is dependent on the oscillation frequency and is finite when the frequency tends to infinity.     

Heat Transfer and Pressure loss of a very shallow fluidized-bed heat exchanger Part 1. Experiment with a single row of tubes

A novel gas fluidized-bed heat exchanger with a very small static bed height has been developed for a heat-exchanging system using a low-pressure fan. This fluidized bed is composed of a multislit distributor, a single row of 8 mm diameter tubes, and glass beads 48–195 μm in diameter. The measured performance of heat transfer is excellent and that of fluidization is satisfactory, in spite of the static bed height being as small as 13 mm. In the best case, the test fluidized bed exhibited a heat transfer performance comparable to that of a conventional fluidized bed with a perforated plate distributor and a static bed height of 150 mm, and showed one-fourteenth the pressure loss.     

Fluid flow and heat transfer in microchannels with rectangular cross section

Forced convective heat transfer coefficients and friction factors for flow of water in microchannels with a rectangular cross section were measured. An integrated microsystem consisting of five microchannels on one side and a localized heater and seven polysilicon temperature sensors along the selected channels on the other side was fabricated using a double-polished-prime silicon wafer. For the microchannels tested, the friction factor constant obtained are values between 53.7 and 60.4, which are close to the theoretical value from a correlation for macroscopic dimension, 56.9 for D _h  = 100 μm. The heat transfer coefficients obtained by measuring the wall temperature along the micro channels were linearly dependent on the wall temperature, in turn, the heat transfer mechanism is strongly dependent on the fluid properties such as viscosity. The measured Nusselt number in the laminar flow regime tested could be correlated by which is quite different from the constant value obtained in macrochannels.     

Theoretical investigation on thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger

Based on the heat transfer characteristics of absorber plate and the heat transfer effectiveness-number of heat transfer unit method of heat exchanger, a new theoretical method of analyzing the thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger has been put forward and validated by comparisons with the experimental and numerical results in pre-existing literature. The proposed theoretical method can be used to analyze and discuss the influence of relevant parameters on the thermal performance of heat pipe flat plate solar collector.     

Heat transfer from the heated concave wall of a return bend with rectangular cross section

The characteristics of the turbulent heat transfer along the heated concave walls of return bends which have rectangular cross sections with large aspect ratio have been examined for various clearances of the ducts in detail. The experiments are carried out under the condition that the concave walls are heated at constant heat flux while the convex walls are insulated. Water as the working fluid is utilized. Using three kinds of clearance of 9, 34, and 55 mm, the Reynolds number in the turbulent range are varied from 5×10³ to 8×10⁴ with the Prandtl numbers ranging from 4 to 13. As a result it is elucidated that both the mean and the local Nusselt numbers are always greater than those for the straight parallel plates or for the straight duct, respectively. This is attributed to Görtier vortices, which are visualized here. It is also found that the more the clearance increases, the more both the local and the mean Nusselt numbers increase. Correlation equations for the mean and the local Nusselt numbers are determined in the range of parameters covered. Introducing the Richardson number, it appears that the local Nusselt number,Nu _x , may be described as the following equation:Nu _x =447.745 ·Re _x ^1.497 ·De _x ^?1.596 ·F ^0.960 ·Pr ^0.412     

Secondary flow structures in developing viscoelastic fluid flow through curved ducts with square cross section

Meccanica - The present paper numerically investigates viscoelastic fluid flow in the developing flow regime through both straight and 90-degree curve ducts. The aim is to investigate the effects...     

Convective boiling in a parallel microchannel heat sink with a diverging cross section and artificial nucleation sites

To develop a highly stable microchannel heat sink for boiling heat transfer, three types of diverging microchannels (Type 1, Type 2 and Type 3) were designed to experimentally investigate the effect of different distributions of artificial nucleation sites (ANS) on the enhancement of flow boiling heat transfer, in 10 parallel diverging microchannels with a mean hydraulic diameter of 120 μm. Water was used as the working fluid with mass flux, based on the mean cross section area, ranging from 99 to 297 kg/m² s. The Type-1 system did not contain any ANS; the Type-2 system contained ANS distributed uniformly along the downstream half of the channel; and the Type-3 system contained ANS distributed uniformly along the entire channel. The ANS are laser-etched pits on the bottom wall of the channel and have a mouth diameter of approximately 20-22 μm, as indicted by the heterogeneous nucleation theory. The results of the present study reveal that the presence of ANS for flow boiling in parallel diverging microchannels significantly reduces the wall superheat and enhances the boiling heat transfer performance. The Type-3 system shows the best boiling heat transfer performance.     

Experimental study on forced convection heat transfer characteristics in a converging diverging heat exchanger channel

The convective heat transfer characteristics in a periodic converging–diverging heat exchanger channel are investigated experimentally. Experiments were performed for Prandtl number 0.7, for corrugation angle of 30°. In order to determine the channel having the best performance, the channels also compared by considering the flow area goodness factor.     

Forced convection freezing characteristics along the convex wall of a return bend with rectangular cross section

An experimental study was made of the forced convection freezing characteristics on the convex wall of a return bend with a rectangular cross section. Observations were carried out for duct heights of 17 and 30 mm, a duct width of 300 mm, and a radius of curvature of 159 mm. The convex wall temperature was uniformly maintained below the freezing temperature of water, and the concave wall was insulated. It was found that a stepwise ice layer forms on the convex wall of a return bend and that the step position at the steady-state condition is closely dependent on both the water flow velocity and the cooling temperature ratio.     

Slug flow heat transfer in ducts of regular polygonal cross section

Nusselt numbers for slug-flow heat transfer in regular polygonal ducts for the case of uniform wall temperature have been calculated by means of the point-matching method. The Nusselt values vary from 4.38 for the triangular duct up to 5.78 for the circular duct.

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Zusammenfassung Nusselt-Zahlen für den Wärmeübergang bei Kolbenströmung in Kanälen mit regelmäßigem Vieleckquerschnitt wurden mittels des Point-Matching-Verfahrens für den Fall gleichförmiger Wandtemperatur bestimmt. Die erhaltenen Nusselt-Zahlen variieren von 4,38 für den Dreieckkanal bis 5,78 für das Kreisrohr.

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Nomenclature C _n coefficients - D _e equivalent or hydraulic diameter (=2R for the regular polygon) - I number of angular zones - J _Np Bessel function of the first kind of orderN _p - j ₀=2.404825 zero of the Bessel function of the first kind of order zero - N number of vertices - N _u Nusselt number - p integer - Pe Peclet number - R radius of the inscribed circle of the polygon - r radial coordinate - T temperature - u velocity - z axial coordinate - thermal diffusivity - dimensionless temperature - =4z/D _e Pe dimensionless axial coordinate - _n eigenvalue - =r/R dimensionless radial coordinate - _n eigenfunction - angular coordinate