•Low-pass filters can approximate the dynamics of heat convection in porous media.•The forced response remains linear at low reynolds numbers.•Increases in reynolds number increase the nonlinearity ...in a non-monotonic way.•Changes in porosity can alter linearity of the forced convection response.
An increasing number of technologies require prediction of unsteady forced convection in porous media when the inlet flow is unsteady. To gain further insight into this problem, the unsteady equations of continuity, Navier Stokes and energy are solved within the pores formed by several cylindrical flow obstacles. The system is modulated by sine waves superimposed on the inlet flow velocity, and the spatio-temporal responses of the flow and temperature fields are calculated. The results are then utilised to assess the linearity of the thermal response represented by the Nusselt number on the obstacles. It is shown that for linear cases, a transfer function can be devised for predicting the dynamic response of the Nusselt number. It is further argued that such a transfer function can be approximated by a classic low-pass filter which resembles the average response of the individual obstacles. This indicates that there exists a frequency threshold above which the thermal system is essentially insensitive to flow modulations. The results also show that changes in Reynolds number and porosity of the medium can push the dynamic response of the system towards non-linearity. Yet, there appears to be no monotonic change in the linearity of the response with respect to the Reynolds number and porosity. In general, it is found that for low Reynolds numbers, the dynamics of heat convection can be predicted decently by taking a transfer function approach. The findings of this study can enable further understanding of unsteady forced convection in porous media subject to time-varying inlet flows.
•Lattice Boltzmann simulation research on straight channel with extended surface.•CuO/water, Al2O3/water and TiO2/water nanofluids at volume fraction of 0, 0.01, 0.03 and 0.05%.•Low Reynolds number ...simulations were considered due to the limitation of LBM.•Rate of heat transfer is decrease by increasing the spacing between the extended surfaces.•CuO/water nanofluid performed better than other nanofluids.
Research on nanofluids for heat transfer augmentation has received a great attention from many researchers. Recently, many numerical works have been conducted to examine their applicability in predicting heat transfer with nanofluids. In the present study, a two-dimensional (2D) lattice Boltzmann method (LBM) was applied for numerical simulation of forced convection in a channel with extended surface using three different nanofluids. The predicted were carried out for the laminar nanofluid flow at low Reynolds number (10 ⩽ Re ⩽ 70), nanofluid concentration (0.00 ⩽ φ ⩽ 0.050), different geometric parameter (0.2 ⩽ A = l/H ⩽ 0.8) and relative height of the extended surfaces (0.05 ⩽ B = h/H ⩽ 0.35). The results indicated that the average Nusselt number increases when the nanofluid concentration increased from 0% to 5%. Moreover, the effect of the nanofluid concentration on the increasing of heat transfer is more noticeable at higher values of the Reynolds number. It is concluded that the use of extended surfaces can enhance the rate of heat transfer for certain arrangements. We also found that the nanofluid with CuO nanoparticles performed better enhancement on heat transfer compared Al2O3/water and TiO2/water nanofluids.
•Newly developed solar dryer reduces the drying time of stevia leaves by 62%.•Exergy efficiency of solar dryer and drying process are estimated.•Overall solar dryer efficiency was estimated as ...33.5%.•Quality of stevia leaves dried in the solar dryer is better than open sun drying.•Estimated payback period of the newly developed dryer is 0.65 yr.
In the present work, experimental investigations carried out on drying of Stevia leaves in a newly developed solar dryer of mixed mode forced convection type (MFSCD) and open sun drying (OSD) are presented. Experiments have been performed under the average solar radiation of 567 W/m2, ambient temperature of 30 °C and drying air flow rate of 0.049 kg/s. The safe (final) moisture content of stevia leaves 0.053 (d.b) has reached in 330 min and 870 min of drying time in MFSCD and OSD, respectively. The overall dryer efficiency and average exergy efficiency of the MFSCD were found as 33.5% and 59.1%, respectively. Quality analyses were carried out for fresh, open sun-dried and solar dried stevia leave samples. It was found that the anti-oxidant and the flavonoids were rich in solar dried samples compared to that of OSD samples. The color preservation is good in solar dried samples compared to OSD. Sensory analysis (flavor, aroma and taste) carried out on stevia leaves indicated that the solar dried stevia leaves provided better score compared to OSD samples. The estimated payback period of the newly developed dryer was found as 0.65 yr.
•Thermal boundary layer thickness decreases with an increase in the Kelvin forces.•The thermal plume appears over the outer layer.•As the Reynolds number increases the absolute value of the stream ...function increase.•Thermal plume disappears as the Hartmann number increases.•As the magnetic number increases the main eddies turn into four smaller eddies.
Since advective transport in a ferrofluid can be controlled by using an external magnetic field, magnetic nanofluid (ferrofluid) has various applications to heat transfer processes. Unlike free or forced convection, Ferrohydrodynamic convection is not yet well described. In the literature we see papers with constant magnetic fields; but the assumptions are not accurate, since the fields do not comply with the Maxwell’s equations of electromagnetism. In this study, forced convection heat transfer in a semi annulus lid under the influence of a variable magnetic field is studied. The enclosure is filled with ferrofluid (Fe3O4–water). Control Volume based Finite Element Method (CVFEM) is used to solve the governing equations considering both Ferrohydrodynamic (FHD) and Magnetohydrodynamic (MHD) effects. It is assumed that the magnetization of the fluid is varying linearly with temperature and magnetic field intensity. The effects Reynolds number, nanoparticle volume fraction parameter, magnetic number arising from FHD, and Hartmann number arising from MHD are analyzed. Obtained results indicate that the effects of Kelvin forces are more pronounced for high Reynolds number. Heat transfer enhancement has direct relationship with the Reynolds number and the magnetic number; while it has inverse relationship with the Hartmann number.
•Forced convection heat transfer in pin fin plates of different geometries with equal surface area.•Triangle staggered pin plate heat exchangers reached the maximum Nu numbers.•The staggered ...configuration of the pin fin plates demonstrated high performance in heat transfer.•The highest-pressure drop was in triangular and elliptical pin plate heat exchangers.
In this study, the effects of forced convection heat transfer on the surfaces of pin plate heat exchangers with different geometries modified to the heat exchanger form were experimentally investigated. A total of 8 pin fin plate heat exchangers, four of which are staggered and the other four are in-line, were used in the experiments. The heat transfer surface areas of the pin plate heat exchangers are produced equally. The only difference is their geometry, which is produced in four different shapes: triangle, circle, ellipse and square. The experiments were carried out at air velocities of 1 to 6 m/s with 1 m/s increments and at 10 W and 50 W input powers with 10 W increments. The staggered configuration of the pin fin plates demonstrated high performance in heat transfer. The study found that heat exchangers with cornerless pin plates have lower heat transfer compared to those with cornered pin plates. The pressure drops increased as the velocity of the coolant air increased, and the highest-pressure drop was observed in staggered triangular and elliptical pin plate heat exchangers. This research is considered innovative as it involves an experimental comparison between staggered and in-line pin plate heat exchangers in four different geometries, all with equal surface area.
This study advances the detection of bacteria at low concentrations in single-entity electrochemistry (SEE) systems by integrating forced convection. Our results show that forced convection ...significantly improves the mass transfer rate of electrolyte, with the mass transfer coefficient demonstrating a proportional relationship to the flow rate to the power of 1.37. Notably, while the collision frequency of E. coli initially increases with the flow rate, a subsequent decrease is observed at higher rates. This pattern is attributed to the mechanics of cell collision under forced convection. Specifically, while forced convection propels cells towards the ultra-microelectrode (UME), it does not aid in their penetration through the boundary layer, leading to cells being driven away from the UME at higher flow rates. This hypothesis is supported by the statistical analysis of collision data, including signal heights and rise times. By optimizing the flow rate to 2 mL/min, we achieved enhanced detection of E. coli in concentrations ranging from 0.9 × 107 to 5.0 × 107 cells/mL. This approach significantly increased collision frequency by elevating the mass transfer of cells, with the mass transfer coefficient rising from 0.1 × 10−5 m/s to 0.9 × 10−5 m/s. It provides a viable solution to the challenges of detecting bacteria at low concentrations in SEE systems.
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•Enhanced SEE with forced convection for high efficiency cell detection.•Electrolyte mass transfer coefficient scales with flow rate to the power of 1.37.•Unlike electrolyte, E. coli collision frequency rises then drops with flow rate.•The mechanics of cell collision under forced convection is studied.
•Double-pipe heat exchanger with flat inner pipe was investigated numerically.•At Repipe < 7000, flat inner pipe benefits to both thermal and flow performance.•At Repipe < 7000, reduction of Ar can ...cause up to 16.8% performance improvement.•At Repipe > 7000, circular shape is still recommended for design of inner pipe.•At high Repipe, flat inner pipe can bring in severe drop on overall performance.
Clear understanding of the characteristics on convective heat transfer in simple-configuration double-pipe heat exchangers will add great value to design of heat exchangers. In this work, an important while less investigated problem, i.e., turbulent forced convective heat transfer inside novel double-pipe heat exchangers with flat inner pipes, was investigated through CFD. The numerical prediction was first validated with available data. Then, effects of inner pipe geometry on flow and heat transfer were investigated. It was mainly found that effects of inner pipe geometry on thermal characteristics heavily depend on the value of Repipe. For Repipe < 7000, using flat inner pipes with small aspect ratio is beneficial to overall heat transfer, thermal effectiveness, and performance index. About 2.9%, 2.7%, and 16.8% increases in overall convective heat transfer coefficient, thermal effectiveness, and performance index, respectively, were achieved for a flat inner pipe with aspect ratio of 0.37. However, for Repipe > 7000, circular inner pipe significantly outperforms than flat inner ones. These results imply that at low Reynolds numbers, utilization of flat inner pipes with small aspect ratio is preferred to performance improvement for double-pipe heat exchangers, while circular inner pipe is still the best choice at high Reynolds numbers.
Dynamic study of frost formation on cryogenic surface Sun, Biao; Ghatage, Swapnil; Evans, Geoffrey M. ...
International journal of heat and mass transfer,
April 2020, 2020-04-00, 20200401, Letnik:
150
Journal Article
Recenzirano
•The dynamic process of frost formation on cryogenic surface is investigated.•The multiphase CFD model is developed to study the frosting behaviour at different conditions.•The temperature effect is ...evaluated with a range from refrigeration to cryogenic conditions.•Conditions of natural and forced convections are compared under cryogenic temperature.
In this study, a multiphase computational fluid dynamics (CFD) model is developed to investigate the heat and mass transfer phenomenon of frost formation under cryogenic conditions. It is observed from experimental studies that frost formation on a vertical cryogenic surface showed different behaviours compared to refrigeration conditions. Through comparing different empirical correlations of mass transfer, it is found that the Rans–Marshall correlation is suitable to be used to account for frost formation under cryogenic conditions. The simulation results show a good agreement with the experimental study in terms of temperature profile. The mass transfer rate is taken as the parameter to characterise the differences of frost formation under different conditions. Surface temperatures ranging from refrigeration to cryogenic are investigated in order to understand the systematic change of frosting behaviour. Conditions of forced convection at different air velocities are studied to evaluate the mass transfer process. The methodology and results discussed in this study can provide in-depth understandings of mass transfer phenomenon at cryogenic conditions.
•Five types of open-cell copper foams with different porosities were selected.•The flow and heat transfer performance were studied experimentally and numerically.•The numerical simulation considered ...local thermal non-equilibrium.•Experiments and simulations agreed on thermal resistance and pressure drop.
A metal foam with an open-cell structure is a type of material with low flow resistance, high specific surface area, and strong fluid mixing ability. Open-cell metal foams have broad application prospects in electronic component cooling, multiphase heat exchangers, and compact heat exchangers for aerospace applications. This paper presented experimental and numerical analyses of the flow and heat transfer characteristics of five different copper foams under forced air convection. The pores per inch (PPI) of selected foams were 10, 20, 30, 40, and 60, with porosities ranging from 0.968 to 0.973. Analysis of the collected heat transfer and pressure drop data yielded the overall heat transfer coefficient, unit pressure drop, normalized average wall temperature, inertia coefficient, and resistance coefficient. The influence mechanisms of the porosity and flow velocity on heat transfer were analyzed and discussed. The numerical simulation and experiment fitted well. The results showed that increasing porosity led to a significant increase in heat transfer coefficient and unit pressure drop. The 60 PPI foam brought the maximum pressure drop while achieved the minimum thermal resistance.
•A fast numerical method for forced convection transient conjugate heat transfer has been developed.•A new dimensionless number is proposed to determine the moment of suspension of the flow ...field.•The initial error of the quasi-steady algorithm is optimized by choosing a high fluid development stage.•The computational speed is increased while maintaining a certain level of computational accuracy.
In this study, a fast numerical method for solving the forced convection conjugate heat transfer problem was developed. The method first proposes a new dimensionless number (Fs) that represents the degree of influence of convection on the temperature field in the flow field. It delineates the stage of development of the flow field by monitoring the change in Fs to determine the moment when the flow field suspends updating and improve the computational efficiency of the transient temperature field. The accuracy of the algorithm is verified by taking the fluid–solid conjugate heat transfer under forced convection conditions as an example, which can accurately capture the changes of the flow field for a given monitoring step number of 100, and classify the flow field into E3, E4 and E5 development stages according to the judgment criteria. The results show that the higher development stages correspond to smaller levels of root mean square error (RMSE) of monitoring point temperatures within 3600 s of physical simulation time, and stages E3, E4, and E5 can reach the levels of E-2, E-3, and E-4, which are 3.4, 3.3, and 3.1 times faster than the traditional coupled calculations, respectively. The algorithm is still applicable at variable time steps, but it will require a higher number of determinations compared to a fixed step. The initial error of the quasi-steady algorithm can be reduced from the E-1 level to E-4 by choosing a higher stage of development. Finally the algorithm is tested under a variety of conditions by varying the inlet temperature and flow rate and is found to be robust to both device warming and cooling.
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