An overview on heat transfer augmentation using vortex generators and nanofluids: Approaches and applications

Longitudinal vortex generator applied to heat transfer enhancement of a flat plate solar water heater

As the longitudinal vortex is known to be effective in enhancing heat transfer, a three-dimensional unsteady numerical analysis has been made especially for the flow and thermal fields in a unit element of flat tube heat exchanger with vortex generator (VG) in so-called middle Reynolds number range. Both staggered and in-lined arrangements of the tubes with VG are considered and results obtained from the case with VG are compared with those without VG. The study was aimed at the influence of Reynolds number and some geometrical parameters on the heat transfer and the pressure drop. Moreover, as little research has been considered the interaction between transverse vortices and longitudinal vortices in the literatures, the effect is also investigated. It is found that the longitudinal vortex plays an important role in enhancing the local heat transfer by exchanging the fluid from the tube surface region to the fresh fluid of the main flow region and lasts over long distances. Moreover, the longitudinal vortices restrain unsteady transverse vortex shedding. As a result, heat transfer rate for the flat tube with VG case is larger compared to that without VG case. From the view point of pressure drop, increase in pressure drop for the case with VG is not so much larger due to the restraint of transverse vortex shedding. Finally, heat transfer performance becomes higher for the flat tube with VG case compared to that without VG case for the same pumping power.

In the present study, a commercial finite volume package ANSYS FLUENT 14.5 has been used to determine thermo-hydraulic performance of two different vortex generator (VG) configurations, namely, flow-up and flow-down, located in triangular ducts. A comprehensive literature survey on flow-up and flow-down configurations has shown that there are completely opposite results obtained in terms of performance of the two configurations. In some studies, flow-up configuration showed better performance characteristics over flow-down configuration while in other studies flow-down configuration was found better. This has been motivated the present authors to make a study covering the two VG configurations to find out which one has a better performance over the other one. Therefore flow and thermal fields together with the secondary flow induced by the two VG configurations have been analyzed extensively. The geometry comprised of double row of VGs located on the slant surfaces of equilateral triangular ducts. The angle of inclination of the VGs made with the flow direction is 30°. The results show that flow-down configuration has a 40.65% higher thermo-hydraulic performance over the flow-up configuration for Re=5000. It is also shown that the heat is transferred at a further downstream distance allowing in a reduction in the number of VGs to be used in of flow down case.

3D numerical investigation of flow and heat transfer characteristics in smooth wavy fin-and-elliptical tube heat exchangers using new type vortex generators

The performance of a compressor cascade is considerably influenced by secondary flow effects, like the cross flow on the end wall as well as the corner separation between the wall and the vane. An extensive experimental study of vortex generator application in a highly loaded compressor cascade was performed in order to control these effects and enhance the aerodynamic performance. The results of the study will be used in future projects as a basis for parameterization in the design and optimization process for compressors in order to develop novel nonaxisymmetric endwalls as well as for blade modifications. The study includes the investigation of two vortex generator types with different geometrical forms and their application on several positions in the compressor cascade. The investigation includes a detailed description of the secondary flow effects in the compressor cascade, which is based on numerical and experimental results. This gives the basis for a specific approach of influencing the cascade flow by means of vortex generators. Depending on the vortex generator type and position, there is an impact on the end wall cross flow, the development of the horse shoe vortex at the leading edge of the vane, and the extent of the corner separation achieved by improved mixing within the boundary layer. The experiments were carried out on a compressor cascade at a high-speed test facility at DLR in Berlin at minimum loss (design point) and off-design of the cascade at Reynolds numbers up to Re = 0.6 × 106 (based on 40-mm chord) and Mach numbers up to M = 0.7. At the cascade design point, the total pressure losses could be reduced by up to 9% with the vortex generator configuration, whereas the static pressure rise was nearly unaffected. Furthermore, the cascade deflection could be influenced considerably by vortex generators and also an enhancement of the cascade stall range could be achieved. All these results will be presented and discussed with respect to secondary flow mechanisms. Finally, the general application of vortex generators in axial compressors will be discussed.

A 180-deg turning channel can be seen in many heat exchange systems, and a strong three-dimensional flow phenomenon will occur at the turning point. In order to strengthen the heat transfer inside the turn and considering the lack of relevant enhanced endwall heat transfer approach, the present study innovatively demonstrates the measures of enhanced internal endwall heat transfer using arrays of delta-winglet and conical vortex generators. The studied parameters of the vortex generators include aspect ratio (AR), spacing, and angle of attack, etc. The experimental results indicate that the longitudinal vortex generators have more advantages in increasing the heat transfer than the transverse conical vortex generators in certain configurations with the same order of magnitude of the increased area. Besides, transverse conical vortex generators are more helpful to improve the endwall heat transfer uniformity than longitudinal vortex generators. The array of longitudinal vortex generators with 45-deg angle of attack, 0.02 m pitch, and 2 aspect ratio provides the highest thermal performance for all studied cases, indicating that the longitudinal delta-winglet vortex generators can induce near-wall flow turbulence as well as convective heat transfer of the endwall with a modest pressure loss penalty. For this optimally designed endwall, the Nusselt number is augmented by up to 35%. From the perspective of energy saving, the present studied vortex generators provide a superior thermal performance at identical pumping power, which can be useful in the future design of endwalls in sharp-turn channels.

Numerical investigation for the optimal placements of rectangular vortex generators for improved thermal performance of fin-and-tube heat exchangers

This paper deals with a three dimensional numerical study of heat transfer and pressure drop for fin and tube bank heat exchanger with a vortex generator. A new type of vortex generator, curved semi-wavy vortex generators (CSWVGs) is introduced and compared its performance. The computational domain consists of a small portion between two adjacent fins and air is used as the working fluid. The vortex generators are punched out from the fin surface. the analysis show that with the use of new vortex generators convective heat transfer rate has been increased by increasing the solid to fluid interaction through delaying the boundary layer separation, directing the working fluid to the wake region and bringing vortices into the working fluid. The most favourable position and geometric parameter of the new CSWVGs are found by studying the average Nusselt number and friction factor at various Reynolds ranging from 800 to 2000.

The relative positions of leading edges and trailing edges of rectangular vortex generators (VG) in the winglet form play a crucial role in modifying thermal and hydraulic boundary layers thereby affecting the heat transfer augmentation and pressure drop in a sudden expansion channel having expansion ratio (ER) 2:1. A numerical simulation was carried out for solving the momentum and energy equations of a three dimensional vortex- laminar flow (Reynolds number < 190) with a finite volume method based commercial code FLUENT 16.2. Reynolds number, angle of attack and VG configuration have been considered as influential parameters affecting the thermo-hydraulic performance of the sudden expansion channel due to the combined effect of primary recirculation and secondary vortex flow. The heat transfer enhancement along with corresponding pressure drop penalty was compared for the toe-out and toe-in configurations at various Reynolds (Re) numbers and angle of attacks (β). Eventually irrespective of configurations, rectangular winglet VGs are found to have better overall performance at β = 30° and β = 45° in both toe-out and toe-in configurations due to strong secondary vortex flow.

The numerical simulation was carried out to investigate mechanism of the heat transfer enhancement in the fin-and-tube heat exchangers. As known, the vortex generators (VGs) were widely used to improve the thermal performance with bad flow resistance characteristics and led to bad comprehensive performance. This paper aims to expound the mechanism of thermal hydraulic characteristics and explore the effect of VGs position on the comprehensive performance. Three types of fins (type 1, type 2, and type 3) were discussed in this paper. The j factor, f factor, and performance evaluation (PEC) of three types of VGs in different positions were discussed and compared. Based on the numerical results, a detailed description of the effect of three types of VGs on the heat transfer performance and flow resistance characteristics was presented at different Reynolds number in the range between 1300 and 2000. In addition, local velocity distribution, local temperature distribution, and local pressure drop distribution were analyzed and discussed. And the effect of VG angle on the thermal performance and flow resistance was presented. It can be concluded that the main heat transfer occurred in the region before the tube, and the wake region behind the tube was harmful to improve the thermal performance and reduce the flow resistance. Besides, VG in the wake region was obviously beneficial to the enhancement of the thermal performance with less energy loss.

This investigation stems from the area of augmentation of heat transfer by generating streamwise longitudinal vortices. The vortex generators are arranged in a common-flow-up configuration. Existing air-cooled condensers in geothermal power plants use fin-tube heat exchangers with circular tubes. The heat exchangers are huge, and often the cost of the condensers is more than one-third of the plant cost. The size of the condensers can be reduced through enhancement of heat transfer from fin surfaces. The enhancement strategy involves introduction of strong swirling motion in the flow field. The swirl can be generated by the longitudinal vortices. In this study, the longitudinal vortices are created by delta winglet-type vortex generators, which are mounted behind the tubes. An element of a heat exchanger has been considered for detailed study of the flow structure and heat transfer analysis. Biswas and colleagues have obtained significant enhancement of heat transfer by deploying the winglet pair behind each tube. In this study, a novel technique (Torii and colleagues [2]) has been utilized for the enhancement. The winglets are placed with a heretofore unused orientation for the purpose of augmentation of heat transfer. This orientation is called the common-flow-up configuration. The proposed method causes significant separation delay, reduces form drag, and removes the zone of poor heat transfer from the near wake of the tubes. The analyses of flow and heat transfer in the proposed configuration have been accomplished through a numerical solution of complete Navier-Stokes and energy equations.

Numerical study of laminar flow and heat transfer characteristics in the fin side of the intermittent wavy finned flat tube heat exchanger

Experimental investigation on thermoelectric generator with non-uniform hot-side heat exchanger for waste heat recovery

Various technologies have been developed to enhance the heat transfer. Vortex generator (VG) is one of the passive techniques which can change the flow behavior and ultimately enhances the heat transfer performance. Delta winglet (DW) vortex generator can create longitudinal and horseshoe vortices which do not decay until further downstream and consequently increase heat transfer coefficient with comparatively lower pressure drop. With this vortex generator, it is expected to have higher Nusselt number with some increase of friction factor. Therefore, this study is to study the effect of pitch ratio (PR) and attack angle (B) of DW vortex generator to increase the thermal performance of heat exchanger. Four delta winglets are attached into a ring. Those rings attached with VGs will be used to investigate the influence of different parameters to heat transfer performance. In this study VGs were placed inside a circular copper tube and the heating coil was wrapped up around the outer surface of the copper tube to generate a constant heat flux condition. The experimental setup consists of a blower, orifice meter, flow straightener, calm/flow developing section and test section. The results show the friction factor, Nusselt number, and Thermal Performance Enhancement. It increases the thermal performance due to the formation of longitudinal vortex inside the circular tube. Pitch ratio and attack angle seem to have significant impact on the flow and heat transfer. The Pitch ratio of 1.6 have the highest impact on both (f/f0) and (Nu/Nuo) followed by attack angle. Smoke flow visualization technique was used to study flow behavior and flow structures.

We have performed 3-d hydrodynamic Rayeligh-Taylor simulations with initial perturbations with two modes to clarify nonlinear dynamics of the bubbles. By imposing two modes with different initial amplitudes, well defined bubbles with three different sizes, small, medium and large, can be formed. We pay attention to the generation of not only transverse vortex, perpendicular to the gravity, but also the longitudinal one, and relation between the bubble dynamics and the vortex. It is pointed that the longitudinal vortex is generated due to pressure gradient, parallel to the gravity, caused by fluid flow stagnated at the saddle and that three different bubble growth can be understood from the point of vortex dynamics view.