Holger Martin

Heat and Mass Transfer between Impinging Gas Jets and Solid Surfaces

Publisher Summary Heating or cooling of large surface area products is often carried out in devices consisting of arrays of round or slot nozzles, through which air impinges vertically upon the product surface. This chapter presents a comprehensive survey emphasizing the engineering applications and empirical equations, presented for the prediction of heat and mass transfer coefficients within a large and technologically important range of variables. The local variations of the transfer coefficients are based on the experimental data for single round nozzles (SRN), arrays of round nozzles (ARN), single slot nozzles (SSN), and arrays of slot nozzles (ASN). The variation of local transfer coefficients is graphically represented. It also explores how to apply these equations in heat exchanger and dryer design as well as in optimization. The flow field of impinging flow is diagrammatically represented. External variables influencing heat and mass transfer in impinging flow depends on mass flow rate, kind and state of the gas and on the shape, size, and position of the nozzles relative to each other and to the solid surface. The design of high-performance arrays of nozzles is also discussed.

Low peclet number particle-to-fluid heat and mass transfer in packed beds

Abstract For low Peclet numbers most of the experimentally obtained particle-to-fluid heat and mass transfer coefficients in packed beds were found to be some orders of magnitude below the values predicted for a single sphere in cross flow. From theoretical considerations one should expect the transfer coefficients in packed beds to exceed the single sphere predictions as they actually do for higher Peclet numbers. The obvious discrepancy between theory and experiment can be cxplained by a simple model accounting for a nonuniform distribution of the void fraction. The model consists of a packed bed of uniformly sized particles with an average void fraction ψ, where a small part ϕ of the total cross-sectional area f is assumed to have a larger void fraction ψ 2 . Since the same pressure drop applies to both parts of the bed, the superficial velocity will be much larger in the section with the larger void fraction, especially in the range of low Reynolds numbers. Even though in both parts of the packed bed the individual transfer coefficients are taken from a correlation which is based on the single sphere predictions (as it is valid in the range of high Peclet numbers), the apparent overall transfer coefficients for the nonuniform system become much lower, and show the same characteristic variation with Peclet number and the ratio of particle diameter to bed height as the majority of the experimental data.

A theoretical approach to predict the performance of chevron-type plate heat exchangers

Abstract Manufacturers of plate and frame heat exchangers nowadays mainly offer plates with chevron (or herringbone) corrugation patterns. The inclination angleof the crests and furrows of that sinusoidal pattern relative to the main flow direction has been shown to be the most important design parameter with respect to fluid friction and heat transfer. Two kinds of flow may exist in the gap between two plates (pressed together with the chevron pattern of the second plate turned into the opposite direction): the crossing flow of small substreams following the furrows of the first and the second plate, respectively, over the whole width of the corrugation pattern, dominating at lower inclination angles (lower pressure drop); and the wavy longitudinal flow between two vertical rows of contact points, prevailing at highangles (high pressure drop). The combined effects of the longer flow paths along the furrows, the crossing of the substreams, flow reversal at the edges of the chevron pattern, and the competition between crossing and longitudinal flow are taken into account to derive a relatively simple but physically reasonable equation for the friction factor ξ as a function of the angleand the Reynolds number Re. Heat-transfer coefficients are then obtained from a theoretical equation for developing thermal boundary layers in fully developed laminar or turbulent channel flow — the generalized Leveque equation — predicting heat-transfer coefficients as being proportional to (ξ·Re2)1/3. It is shown, by comparison, that this prediction is in good agreement with experimental observations quoted in the literature.

Thermal conductivity of packed beds: A review

Abstract The voluminous literature available on thermal conductivities of packed beds without fluid flow is reviewed. The discussion includes experimental methods as well as theoretical approaches. A classification of predictive models is attempted. One of these models is analysed in detail and recommended for practical use. The predictions of the model are compared with a large amount of experimental data. In the course of this comparison phenomena of physical interest and of technical significance are discussed. Most of the methods presented in this review retain validity for dispersed systems other than gas-filled packed beds and for properties other than the thermal conductivity.

Flow distribution and pressure drop in plate heat exchangers—II Z-type arrangement

Abstract An analytical study is made to calculate the axial velocity and pressure distributions in both the intake and exhaust conduits of plate heat exchangers, the flow distribution in the channels between the plates and the total pressure drop. The analysis shows that there is a general characteristic parameter ( m ) for all the plate heat exchangers, which determines the flow behaviour. When m 2 is positive, the channel flow rates will decrease in the direction of the intake stream. When m 2 is negative, the channel flow rates will increase in that direction. In any case, the flow distribution tends to be uniform for low values of m 2 ( ≤ 0.01). If m 2 is kept equal to zero, the flow distribution will be purely uniform. The sign of m 2 may be controlled by the area ratio of the intake and exhaust conduits respectively, its magnitude is affected by the shape and the number of plates. The results show that very low flow rates or even no fluid flow might occur in some of the channels for large positive or large negative values of m 2 . The analysis might be useful not only for the proper design of plate heat exchangers but also for many other technical systems with manifold flow distribution and collection problems.

Heat transfer between gas fluidized beds of solid particles and the surfaces of immersed heat exchanger elements, part I☆

Abstract After some general remarks about fluidization, and a section on the hydrodynamic behaviour of fluidized beds, the mechanisms of heat transfer between the surfaces of heat exchanger elements and gas—solid fluidized beds are discussed in detail. A theoretical model, presented some years ago, is slightly modified and further developed to improve its applicability within a wide range of variables. The model makes use of some of the basic concepts of molecular kinetic theory as applied to solid particles in a fluidized bed. A complete derivation as well as all the parameters required to apply the model equations are given.

Gas to Gas Heat Transfer in Micro Heat Exchangers

Abstract By the method of mechanical manufacture of microstructures jointly developed by the Karlsruhe Nuclear Research Center (KfK) and Messerschmitt-Bolkow-Blohm (MBB) very compact cross-flow micro heat exchangers have been fabricated whose active volumes are 1 × 1 × 1 cm3 and whose typical channel cross-sections are 75 × 90 μm2. Measurements with nitrogen, argon and helium as heat transfer fluids were performed on two copper and stainless steel finned plate heat exchangers. With the stainless steel heat exchanger overall heat transfer coefficients up to 1700 W m−2 K−1 have been attained in the range investigated using helium as the fluid. For low flow rates greatly reduced overall heat transfer coefficients have been measured compared with the values to be anticipated for laminar flow. In some ranges higher heat transfer rates have been achieved with the stainless steel heat exchanger compared with the system made from copper. This leads to the conclusion that the heat transfer behavior for low gas flow rates is largely determined by heat conduction in the longitudinal direction within the wall and fin materials. With a homogeneous model the influence of longitudinal heat conduction in the stationary walls can be explained and is in fairly good agreement with the experimental data.

Calculations of Steady and Pulsating Impinging Jets—An Assessment of 13 Widely used Turbulence Models

Computational fluid dynamics (CFD) calculations of impinging jets have been performed with 13 widely spread Reynolds-averaged Navier-Stokes (RANS) turbulence models using commercial CFD software. It was the aim of these calculation to assess how heat transfer and flow structure can be predicted at different Reynolds numbers and different nozzle-to-plate distances. The only model able to predict correctly the laminar–turbulent transition occurring at small nozzle-to-plate distance was the SST k − ω model (“transitional flow option”). Most of the other models can only satisfactorily predict heat transfer in the turbulent wall jet region. When the SST k − ω model with transitional flow option is applied for calculation of pulsating jets, the tendencies are predicted correctly.

New correlations for mixed turbulent natural and forced convection heat transfer in vertical tubes

This work deals with mixed natural and forced turbulent convection heat transfer in vertical tubes. It summarizes the experimental results available in literature for both aiding and opposing flow conditions and presents own experimental results that concentrated on the influence of length-to-diameter ratio and of heat and mass flux directions on heat transfer in vertical tubes. Finally, it presents two new empirical correlations that represent experimental results better than all existing correlations in literature, since they take into account the laminarization of turbulent flow that occurs during aiding mixed convection. They predict experimental results from literature and from this work with an accuracy of better than ± 20%.

Fusion-like behaviour of wood pyrolysis

Abstract The results of an experimental study of the melting rates of rods of ice, paraffin and Rilsan (polyamide 11) pressed against a spinning hot disk are reported. The observations are similar to those made with wood rods, the “fusion temperature” of which has been found close to 739 K. These results are in agreement with a theoretical study of the competition between the kinetics of thermal wood decomposition and heat transfer resistances inside and outside the wood. A second conclusion is that because of the heat fluxes that can be transferred in most of the experimental devices, it seems almost impossible to raise wood to temperatures greater than about 800 K; the direct experimental determination of wood pyrolysis reaction rate constants at temperatures above 800 K then appears impossible.