Heat transfer enhancement

Many studies in the open literature were performed to analyze the convective heat transfer enhancement in MHD flows [ 6 , 7 , 8 , 9 ]. Motozawa et al. [ 10 ] studied experimentally the con- vective heat transfer in rectangular smooth duct flow of a magnetic fluid at a Reynolds number of 780. As expected, the local Nusselt increased in the proximity of the external magnetic field. A maximum recorded increase of the Nusselt number was 20%. Considering the difficulty of visualizing the flow, the study did not include an analysis of the flow structure vis-`a-vis the heat transfer enhancement. More specifically, an experimental study on MHD flows using a potassium hydroxide solution (KOH) revealed that for low Reynolds numbers the MHD effects are more prominent due to weaker production of turbulence [ 11 ]. It is also important to men- tion that numerical studies have shown that the pressure drop for electrically insulated ducts is lower than electrically conducting walls due to eddy currents generated near the electrically conducting walls [ 12 ].

a b s t r a c t Heat transfer is a naturally occurring phenomenon that can be greatly enhanced with the aid of vortex generators (VG). Three-dimensional numerical simulations of longitudinal vortex generators are per- formed to analyze heat transfer enhancement in parallel plate-fin heat exchanger. The shear-stress transport (SST) k - u model is adopted to model the flow turbulence. Empirical correlations from the open literature are used to validate empty channel simulations. First, numerical simulations are con- ducted for the classical delta winglet pair (DWP) which is introduced as the reference case in this study. Then, an innovative VG configuration, named inclined projected winglet pair (IPWP), is examined and it shows superior performance relative to the DWP. The IPWP exhibits similar heat transfer rates than that of the DWP but with lower pressure drop penalty due to its special aerodynamic design. The local performance is analyzed based on the streamwise distribution of Nusselt number and friction coefficient criteria in addition to vorticity. This study highlights the different mechanisms involved in the convective heat transfer intensification by generating more vortices using more aerodynamic VG shape while decreasing the pressure drop penalty.

ABSTRACT The nanofluid literature contains many claims of anomalous convective heat transfer enhancement in both turbulent and laminar flow. To put such claims to the test, we have performed a critical detailed analysis of the database reported in 12 nanofluid papers (8 on laminar flow and 4 on turbulent flow). The methodology accounted for both modeling and experimental uncertainties in the following way. The heat transfer coefficient for any given data set was calculated according to the established correlations (Dittus- Boelter’s for turbulent flow and Shah’s for laminar flow). The uncertainty in the correlation input parameters (i.e. nanofluid thermo-physical properties and flow rate) was propagated to get the uncertainty on the predicted heat transfer coefficient. The predicted and measured heat transfer coefficient values were then compared to each other. If they differed by more than their respective uncertainties, we judged the deviation anomalous. According to this methodology, it was found that in nanofluid laminar flow in fact there seems to be anomalous heat transfer enhancement in the entrance region, while the data are in agreement (within uncertainties) with the Shah’s correlation in the fully developed region. On the other hand, the turbulent flow data could be reconciled (within uncertainties) with the Dittus-Boelter’s correlation, once the temperature dependence of viscosity was included in the prediction of the Reynolds number. While this finding is plausible, it could not be conclusively confirmed, because most papers do not report information about the temperature dependence of the viscosity for their nanofluids.

a b s t r a c t Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions in the system. One method involves the use of vortex generators to produce streamwise and transverse vortices on the top of the main flow. This study presents numerical computation results on laminar convection heat transfer in a rectangular channel which bottom wall is equipped with rectangular winglet pair vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible fluid. Three dimensional numerical simulations are performed to study the effect of the generators' roll-angle b on the flow and heat transfer characteristics. Different values of roll- angle b in the range [20
e90
] are considered, while maintaining a constant angle of attack ( a ¼ 30
) for

Dominique Tarlet Laboratoire de Thermocinétique de Nantes (CNRS UMR 6607), Polytech Nantes, rue C. Pauc, BP 50609, 44306 Nantes, France Long Abstract Introduction One of the most promising techniques to enhance convective heat transfer in air is based on the use of electric fields to induce secondary flows (or ionic wind). The ions which are propelled by the Columbic force transfers momentum to neutral fluid molecules which results in the bulk movement of air, causing an interaction between the ionic wind and the primary air flow (EHD combined flow). The EHD combined flow increases the convective heat transfer coefficient on the plate surface [3], [1]. Many questions regarding the optimization of the operating parameters still remain unanswered as reported by [1],[2]. In this context, we are interested in the forced convection heat transfer enhancement by the ionic wind produced by corona discharge (wire to plate) in a channel.

Received: 10 May 2017 / Accepted: 24 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017 Abstract Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions. One method involves the use of vortex generators to produce streamwise and transverse vortices superimposed to the main flow. This study presents numerical computation results of laminar convection heat transfer in a rectangular channel whose bottom wall is equipped with one row of rectangular wing vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible flow. Three-dimensional numerical simulations are performed to study the effect of the angle of attack α of the wing on heat transfer and pressure drop. Different values are taken into consideration within the range 0° < α < 30°. For all of these geometrical configurations the Reynolds number is maintained to Re = 456. To assess the effect of the angle of attack on the heat transfer enhancement, Nusselt number and the friction factor are studied on both local and global perspectives. Also, the location of the generated vortices within the channel is studied, as well as their effect on the heat transfer enhancement throughout the channel for all α values. Based on both local and global analysis, our results show that the angle of attack α has a direct impact on the heat transfer enhancement. By increasing its value, it leads to better enhancement until an optimal value is reached, beyond which the thermal performances decrease.

A multilayer scaling model showed that the overall device per- formance depends on the pumping and power characteristics of the integrated fans as well as the flow [r]

Last but not the least, I would like to thank my family: my father Pramod Kumar Varshney, sister Priyanka Prabhat and good friend Manisha Nadir for supporting me[r]

5.3.2 E ffect of Radial Inlets Sometimes a multilayer device’s performance does not improve significantly with addi- tional fan layers, or the addition of fans pushes the device into an unfavorable operating regime that relies on recirculatory flow. The parabolic increase in the inlet pressure drop tends to render additional fan layers ine ffective beyond a certain number (see Fig. 5-2). Addition of radial inlets lessens the slope of the composite system resistance curve and shifts the operating point to a higher overall volume flow. Sometimes this can have a drastic e ffect on the system’s overall performance. For example, Fig. 5-11 shows the per- formance in heat transfer, pumping power, and volume flow (compared to their respective single-fan values). Compared to Fig. 5-10, which shows the performance for an identical device with only one axial inlet, the difference is striking. In the axial inlet case (the solid lines in Fig. 5-11), the volume flow quickly plateaus and causes the heat sink to rely on recirculation flows for additional heat transfer. By adding a single radial inlet, this same device’s performance changes to that of the middle dashed lines. The volume flow now plateaus at a higher level and saturates at a higher number of fans. Also, the heat trans- fer’s departure from linear scaling does not occur as rapidly. Finally, the effectiveness does not reach unity until 27 layers, which is probably a larger-than-practical heat sink size. Thus, recirculation at the outlet (and the associated risk of reingesting warm air) can be avoided by using radial inlets. The pumping power increases with the addition of radial inlets due to the increase in volume flow. The model results shown in Fig. 5-11 do not include the e ffect of interlayer blockage, so the position of the inlet layers does not have an effect on the results.

* Laboratoire de Thermocinétique, CNRS-UMR 6607 Ecole Polytechnique de l’Université de Nantes, Rue Christian Pauc, BP 50609, F-44306 Nantes Cedex 3 – France ABSTRACT Compact heat exchangers are well known for their ability to transfer a large amount of heat while retaining low volume and weight. The purpose of this paper is to study the potential of using this device as a chemical reactor, generally called a heat exchanger-reactor (HEX reactor). Indeed, the question arises: can these geometries combine heat transfer and mixing in the same device? Such a technology would offer many potential advantages, such as better reaction control (through the thermal aspect), improved selectivity (through intensified mixing, more isothermal operation and shorter residence time, and sharper RTDs), byproduct reduction, and enhanced safety. Several geometries of compact heat exchanger based on turbulence generation are available. This paper focuses on one type: vortex generators. The main objective is to contribute to the determination of turbulent flow inside various geometries by computational fluid dynamics methods. These enhanced industrial geometries are studied in terms of their thermal-hydraulic performance and macro- /micro-mixing ability. The longitudinal vortices they generate in a channel flow turn the flow perpendicular to the main flow direction and enhance mixing between the fluid close to the fin and that in the middle of the channel. Two kinds of vortex generators are considered: a delta winglet pair and a rectangular winglet pair. For both, good agreement is obtained between numerical results and data in the literature. The vortex generator concept is found to be very efficient in terms of heat-transfer enhancement and macro- mixing. Nevertheless, the micro-mixing level is poor due to strong inhomogeneities: the vortex generator must be used as a heat- transfer enhancement device or as a static mixer for macro- and meso-mixing.

Intensive works investigated the influence of several parame- ters related to vortex generators in order to optimize the heat and mass transfer by varying their shapes, number of rows and rel- ative distance from the duct or from each other. For example, Depaiwa et al. [8] experimentally studied the effect of adding ten pairs of rectangular winglet vortex generators (WVG) in a solar air heating system, also taking into consideration the angle of attack by studying two different arrangements, by pointing upstream (PU) or downstream (PD) the flow with various angles (30 °, 45°, 60°). They concluded that the largest angle of attack (60 °) of the PD-WVGs yields the highest increase in Nusselt num- ber and friction factor while the angle of 30 ° in the PU-WVGs showed the best thermal performance among all the cases. In addi- tion, Habchi et al. [9] investigated configurations in which several tab arrays were aligned as in the conventional high efficiency vor- tex system (HEV), alternated from one another or reversed com- pared to the standard HEV geometry. The results showed that the reversed tab arrays give better efficiency in meso- and micro- mixing than the aligned and alternated arrays but show higher pressure drop. Nevertheless, Ma et al. [10] investigated the fluid flow and the convective heat transfer in a rectangular channel with either four longitudinal rigid vortex generators or not. They observed that the RVG could greatly improve the heat transfer rate by about 101% with an increase of pressure drop of only 11% in the laminar flow regime. In the turbulent regime however, the RVG causes heat transfer enhancement of about 87% with an increase of pressure drop of about 100%.

Heat transfer in domed cavities is quite complex owing to the spherical shape and the three-dimensional aspect of the cavity. A few studies have addressed this type of geometry. Laouadi and Atif [1] recently used the numerical control volume approach to investigate the laminar natural convection within domed cavities heated from the outside. They reported that the cavity flow is stable and reaches steady state conditions for the studied cavity gap spacing to dome radius ratios and Rayleigh numbers. Practical correlations for the heat transfer coefficient as a function of the cavity shape and gap spacings were developed from the calculated data. McGowan et al. [2] measured and simulated the

A local heat transfer coefficient may be defined in a similar fashion to Eq. 1 The local Nu is a function of a number of parameters such bank type and geometry, flow Re, pressure gradient, location within bank etc. Figure 1 shows local Nu distributions in the interior of tube banks. It can be seen that the staggered-geometry Nu- distribution is similar to the single cylinder case, with a maximum occurring at φ = 0 ° . For the in-line case, Nu rises to a maximum at the reattachment point around 45 ° . It can also be seen that heat transfer is somewhat higher for both inline and staggered tube banks, than for single cylinders. This is due to increased free- stream turbulence (as a result of preceeding rows) and shear, due to constriction of the flow passages. For most applications the engineer is not concerned with the details of local heat transfer in tube banks and the reader is referred to the reader is referred to _

The gases are continuOUSly transferring heat to walls and object, so that both attain a temperature dependent on the quantity of heat supplied to them.. The better the walls are insulate[r]

laboratory experiment, the effects of the slag composition, the liquid slag temperature, the convection in the liquid, and the composition in the metal sphere are mea[r]

However, we find that although this mode of condensation is readily achievable when condensing working fluids with high surface tension, such as water, even re-ent[r]

Finally, in our knowledge, there is no study looking into the modeling of the coupled conductive and radiative heat transfer from the 3D-microstructure of porous media as a function of the temperature. The purpose of this paper consists in describing the modeling approach and explaining the analytical and numerical resolution of this thermal problem. We have demonstrated how to solve the flash method in such voxelized structures and how to organize the analytical resolution of this problem to compute it in a calculation code giving the transient temperature field in each voxel. Furthermore, we have enumerated all the thermophysical and radiative data introduced in the developed calculation code for each constituent at the voxel scale to carry out our simulations. Finally, we have obtained the first validation results of the code.

Let denote Ω the area of the 2D Representative Volume Element (RVE) of the microcracked media, ∂Ω its outer boundary and u the outward unit normal to ∂Ω ( Fig . 1 a). The macroscopic temperature gradient G (respectively heat flux Q) can be defined as the mean temperature (resp. external heat flux) on the boundary ∂Ω. Under sta- tionary thermal conditions, the macroscopic temperature gradient G (resp. heat flux Q) corresponds to the average of the corresponding microscopic quantity g (resp. q):

FIG. 1. Schematic of the Twente Mass and Heat Transfer Water Tunnel (TMHT). x, y, and z are the spanwise, wall-normal, and streamwise directions, respectively, where each of the measurement sections has dimensions W (width), D (depth), and L (length) in the x, y, and z directions. (a) A side view cross section of the tunnel showing the major components. (b) The front and side profiles of the three exchangeable measurement sections and their respective contractions. (c) A top view of the active turbulent grid showing the 15 independently rotating rods with agitator flaps attached to them, here all in their horizontal position for displaying purposes. (d) The 12 cylindrical heater cartridges in red and the thermal insulation in yellow. (e) Bubble injection provided by five rows of each 28 exchangeable capillaries. The rows can be opened and closed independently from each other. See Sec. II A for a walk-through description.

Heat transfer coefficients for natural water surfaces Williams, G. P. https://publications-cnrc.canada.ca/fra/droits L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.