Abstract
Three‐dimensional flows over backward facing s.tif are analysed by means of a finite element procedure, which shares many features with the SIMPLER method. In fact, given an initial or guessed velocity field, the pseudovelocities, i.e. the velocities that would prevail in the absence of the pressure field, are found first. Then, by enforcing continuity on the pseudovelocity field, the tentative pressure is estimated, and the momentum equations are solved in sequence for velocity components. Afterwards, continuity is enforced again to find corrections that are used to modify the velocity field and the estimated pressure field. Finally, whenever necessary, the energy equation is solved before moving to the next step.
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Abstract
Most compact heat exchangers and heat dissipating components rely on convection enhancement mechanisms that reduce the continuous growth of boundary layers. Usually surface irregularities, in the form of interruptions and/or vortex generators, are introduced in the flow passages. The resulting geometric configurations are periodic in space and, after a short distance from the entrance, induce velocity and thermal fields that repeat themselves from module to module. The numerical models presented here consider the space‐periodicity and allow flows that are stationary at sub‐critical values of the Reynolds number, but become time‐periodic, or quasi periodic, above the critical value of the Reynolds number. Space discretizations are achieved by an equal order finite element procedure based on a projection algorithm. Two‐dimensional schematizations are employed to analyze the effects of surface interruptions and transverse vortex generators, while three‐dimensional schematizations are employed for longitudinal vortex generators.
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G. Croce and P. D’Agaro
A three‐dimensional numerical investigation of flow field and heat transfer in sine‐wave crossed ducts is presented. Numerical simulations are carried out using a finite element…
Abstract
A three‐dimensional numerical investigation of flow field and heat transfer in sine‐wave crossed ducts is presented. Numerical simulations are carried out using a finite element procedure based on an algorithm which shares many features with the SIMPLER finite‐volume method, and utilizes equal order pressure–velocity interpolation functions. Since the flow, after a short entrance regime, reaches the fully developed condition, the computational domain can be reduced to a single periodic element and periodic boundary conditions are assumed at the entrance, the exit and the sides. The thermal performance and the frictional pressure losses of the crossed‐corrugated plates are investigated for different Reynolds number, from steady up to transitional regimes. The evolution from steady to unsteady flow structure is detected and the influence of the unsteadiness on heat transfer and on pressure drop is analysed. Simulations are performed for both air (Pr=0.7) and water (Pr=7) as the flow medium and the dependence of Nusselt number on Prandtl number is investigated.
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Abstract
Purpose
The purpose of this paper is to examine the modeling of simultaneous heat and mass transfer under dehumidifying conditions. Moist air cooling in tube‐fin exchangers is investigated using a finite element technique.
Design/methodology/approach
The model requires the solution of a conjugate problem, since interface temperatures must be calculated at the same time as temperature distributions in adjacent fluid and solid regions. The energy equation is solved in the whole domain, including the solid region, and the latent heat flux on the surfaces where condensation takes place is taken into account by means of an additional internal boundary condition.
Findings
Thermal performances for different Reynolds numbers of a typical two‐row tube‐fin exchanger are numerically analysed, for both in‐line and staggered arrangements of tubes. The results justify the great importance that the ratio between latent and overall rates of heat transfer has in the design of compact heat exchangers.
Practical implications
In this work, the capabilities of the proposed methodology to deal with industrial applications in the field of compact exchangers are outlined.
Originality/value
The paper presents an effective approach to the solution of conjugate conduction and convection problems with simultaneous heat and mass transfer. The formulation is completely general, even if the finite element method is used in the calculations.
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Abstract
The paper adopts a simplified two‐dimensional approach to deal with convective heat and mass transfer in laminar flows of humid air through wavy finned‐tube exchangers. The computational domain is spatially periodic, with fully developed conditions prevailing at a certain distance from the inlet section. Both the entrance and the fully developed flow region are investigated. In the fully developed region, periodicities in the flow, temperature and mass concentration fields are taken into account. The approach is completely general, even if the finite element method is used for the discretizations. In the application section, velocity, temperature, and mass concentration fields are computed first. Then apparent friction factors, Nusselt numbers, Colburn factors for heat and mass transfer, and goodness factors are evaluated both in the entrance and in the fully developed region.
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Three‐dimensional laminar forced convective heat transfer in ribbed square channels is investigated. In these channels, transverse and angled ribs are placed on one or two of the…
Abstract
Three‐dimensional laminar forced convective heat transfer in ribbed square channels is investigated. In these channels, transverse and angled ribs are placed on one or two of the walls to form a repetitive geometry. After a short distance from the entrance, also the flow and the dimensionless thermal fields repeat themselves from module to module allowing the assumption of periodic, or anti‐periodic, conditions at the inlet/outlet sections of the calculation cell. Prescribed temperature boundary conditions are assumed at all solid walls, including the ribs. Pressure drop and heat transfer characteristics are compared for rib angles ranging from 90° (transverse ribs) to 45°, and different values of the Reynolds number. The influence of rib geometries is investigated below and above the onset of the self‐sustained flow oscillations that precede the transition to turbulence. Numerical simulations are carried out employing an equal order finite‐element procedure based on a projection algorithm.
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Joint descriptions of both heat and mass transfer and thermodynamic aspects of air‐cooling applications cannot be easily found in the literature. Numerical analyses are a notable…
Abstract
Purpose
Joint descriptions of both heat and mass transfer and thermodynamic aspects of air‐cooling applications cannot be easily found in the literature. Numerical analyses are a notable exception since suitable physical models and realistic boundary conditions are a prerequisite of accurate simulations. Thus, it is believed that the experience gained with numerical simulations might be of some help also to designers of air‐conditioning and drying systems. This paper seeks to address this issue.
Design/methodology/approach
In the text, the physical implications of governing equations and boundary conditions utilized in numerical simulations are extensively discussed. Particular attention is paid to the thermodynamically consistent definition of latent and sensible heat loads, and to the correct formulation of the heat and mass transfer analogy.
Findings
Comparisons of analytical and numerical results concerning forced flows of humid air over a cooled plate validate the assumptions made in numerical simulations, both for air‐conditioning applications (almost always characterized by low rates of mass convection) and drying applications (almost always characterized by high rates of mass convection).
Originality/value
Finally, with reference to the cold plate problem investigated here, the effects of the suction flow induced by condensation on the Nusselt number are quantified.
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This chapter offers an overview of Indigenous Entrepreneurship (IE) in the national Canadian context and aims to analyze how the diversity among the Aboriginal peoples of Canada…
Abstract
Purpose
This chapter offers an overview of Indigenous Entrepreneurship (IE) in the national Canadian context and aims to analyze how the diversity among the Aboriginal peoples of Canada in society is managed with regard to entrepreneurship.
Findings
Taking into account the scope of diversity, three major dimensions were identified for analysis – (1) the sociocultural dimension, in reference to the worldviews and values of indigenous peoples, (2) the institutional dimension, in reference to the political management of reservations and the Band Council system, and (3) the financial dimension, in reference to the financial opportunities available to indigenous entrepreneurs.
Originality/Value
This chapter’s original contribution rests in its critical analysis of IE in Canada, taking into account the history, the process of colonization and the diversities within the diversity.
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Eirik Sjåholm Knudsen and Lasse B. Lien
The relevance of finance for strategy is probably never greater than during a recession. We argue that the strategy literature has been virtually silent on the issue of…
Abstract
The relevance of finance for strategy is probably never greater than during a recession. We argue that the strategy literature has been virtually silent on the issue of recessions, and that this constitutes a regrettable sin of omission. Recessions are also periods when the commonly held view of financial markets in the strategy literature – efficient, and therefore strategically irrelevant – is particularly misplaced. A key route to rectify this omission is to focus on how recessions affect investment behavior, and thereby firms’ stocks of assets and capabilities which ultimately will affect competitive outcomes. In the present chapter, we aim to contribute by analyzing how two key aspects of recessions, demand reductions and reductions in credit availability, affect three different types of investments: physical capital, R&D and innovation, and human- and organizational capital. We synthesize and conceptualize insights from finance- and macroeconomics about how recessions affect different types of investments and find that recessions not only affect the level of investment, but also the composition of investments. Some of these effects are quite counterintuitive. For example, investments in R&D are both more and less sensitive to credit constraints than physical capital is, depending on available internal finance. Investments in human capital grow as demand falls, and both R&D and human capital investments show important nonlinearities with respect to changes in demand.
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Abstract
By neglecting the influence of tubes, this paper adopts a simplified two‐dimensional approach to deal with laminar convection of air through wavy finned‐tube exchangers. Pressure drop and heat transfer characteristics are investigated in the fully developed region of the flow channels between adjacent fins. The solutions are presented for several space ratios (height over length of a module) and two corrugation angles. They concern laminar flows both below and above the onset of the self‐sustained oscillations that precede the transition to turbulence. Fully developed velocity and thermal fields are computed by imposing anti‐periodic conditions at inlet/outlet sections of a single calculation cell. In the range of Reynolds numbers investigated, Nusselt numbers and friction factors first increase with space ratios (up to a value depending on the corrugation angle), then start decreasing with increasing space ratios.