Compound Heat Transfer Enhancement Research Articles

Parametric study on thermal performance augmentation of TiO2/water nanofluids flowing a tube contained with dual counter twisted-tapes

The study of compound heat transfer enhancement technique using dual counter twisted tapes (DCTs) together with TiO2/water nanofluids was carried out. The dual twisted tapes in form of counter-swirl tape arrangement were utilized at three twist ratios (y/w = 1.5, 2.0 and 2.5). TiO2/water nanofluids having three different volume concentrations (φ = 0.05, 0.10 and 0.15 %) were employed as the testing fluids. The results indicated that heat transfer rate, friction factor and thermal enhancement index rose with decreasing tape twisted ratio and elevating nanofluid concentration. The TiO2/water nanofluids applied in the present work offered higher Nu than pure water (the base fluid) by 7.3–10.0 %. The application of DCT with the smallest y/w of 1.5 and TiO2/water nanofluids with the largest φ of 0.15 vol% led to the highest thermal enhancement index (TEI) of 1.53, under the same pumping power criteria. Additionally, the k-nearest neighbor (k-NN) and artificial neural network (ANN) were developed to predict the thermal enhancement index (TEI). It was found that the k-NN and ANN models provided maximum R2 values of 0.919 and 0.992 f, respectively.

Compound Heat Transfer Enhancement in Dimpled and Sinusoidal Metal Solar Wall Ducts Fitted with Wired Inserts

An improved Metal Solar Wall (MSW) with integrated thermal energy storage is presented in this research. The proposed MSW makes use of two, combined, enhanced heat transfer methods. One of the methods is characterized by filling the tested ducts with a commercially available copper Wired Inserts (WI), while the other one uses dimpled or sinusoidal shaped duct walls instead of plane walls. Ducts having square or semi-circular cross sectional areas are tested in this work.A developed numerical model for simulating the transported thermal energy in MSW is solved by finite difference method. The model is described by system of three governing energy equations. An experimental test rig has been built and six new duct configurations have been fabricated and tested. Air is passed through the six ducts with Reynolds numbers from 1825 to 7300.Six, new, correlations for Nusselt number and friction factor are developed to assess the benefits that are gained from using the WI and the dimpled and sine-wave duct walls. It is found that higher heat transfer rates are achieved using the Dimpled, semi–circular duct with Wired Inserts (DCWI). Also, it is found that Nusselt number and the pressure drop in the DCWI are respectively(44.2% -100%) and (101.27% - 172.8%) greater than those of the flat duct with WI. The improvement in Nusselt number for flat duct with WI is found to be (1.4 – 2) times the values for flat duct with no WI. The results demonstrated that DCWI provides enhancements efficiency value that is higher than those obtained from other types of ducts. The developed MSW ducts have added to local knowledge a better understanding of the compound heat transfer enhancement.

Natural convection compound heat transfer enhancement by discrete rings and the chimney effect in a vertical cylinder

Natural convection is an indispensable heat transfer process considered in the design of advanced autonomous nuclear reactors. The renaissance of inherently safe micro nuclear reactors calls for increased post shut down heat rejection rates by passive means. This study numerically investigated heat transfer enhancement in a heated cylinder by discrete rings and the chimney effect. Five equally spaced rings of varying thicknesses and heights were fitted in the heated channel to effect enhancement by flow interruption. The chimney effect was studied by appending adiabatic chimney extensions of different heights, giving rise to extension ratios of 1.5, 1.6, 1.65, and 1.75. Compound enhancement was achieved by attaching chimney extensions to a ringed channel. Nusselt number ratios, mass flow rates, temperature, and velocity profiles were presented. The results showed that compound heat transfer enhancement could increase Nusselt numbers by between 44 and 54% and lower temperatures by over 30% depending on the chimney height. Furthermore, very short chimney heights would be required in the compound setup in comparison to the tall chimneys in the chimney-only cases, for the same target heat transfer rates. The resulting compactness could greatly influence the design of very small reactors and contribute to improvements of other air-cooled passive systems.

Maximum benefits from the use of enhanced heat transfer surfaces

The paper presents a critical review of the performance evaluation criteria (PEC) developed by Webb (Webb, 1981). These criteria evaluate the benefits that could be obtained in heat exchangers if their surfaces and inserts are designed to enhance the heat transfer, particularly in two-fluid heat exchangers in single-phase flow. This study revealed that the imposed constraint for fixed pumping power in the comparison of the augmented channel with the reference one is a major obstacle to achieving greater benefit. Accordingly, the elimination of the constraint of fixed pumping power and the objective of reduced pumping power decreases substantially the criteria of Webb. A new constraint related to the fixed augmentation entropy generation number has been involved replacing the constraint of fixed pumping power. The pumping power can be left to increase until the augmentation entropy generation number is less than unity. The maximum benefit can be obtained when the augmentation entropy generation number reaches the value of unity. The use of greater in tube heat transfer enhancement (with the use of compound heat transfer enhancement techniques instead of simple ones), and with an enhanced outer tube surface of the augmented exchanger is a useful instrument to increase the benefits, only if the augmentation entropy generation number does not cross the frontier of unity.

Compound Heat Transfer Augmentation of a Shell-and-Coil Ice Storage Unit with Metal-Oxide Nano Additives and Connecting Plates.

Due to the high enthalpy of fusion in water, ice storage systems are known as one of the best cold thermal energy storage systems. The phase change material used in these systems is water, thus it is inexpensive, accessible, and completely eco-friendly. However, despite the numerous advantages of these systems, the phase change process in them is time-consuming and this leads to difficulties in their practical application. To solve this problem, the addition of nanomaterials can be helpful. This study aims to investigate the compound heat transfer enhancement of a cylindrical-shaped unit equipped with double helically coiled coolant tubes using connecting plates and nano additives as heat transfer augmentation methods. Complex three-dimensional numerical simulations are carried out here to assess the best heat exchanger material as well as the impact of various nanoparticle types, including alumina, copper oxide, and titania, and their concentrations in the PCM side of the ice storage unit. The influence of these parameters is discussed on the charging rate and the temperature evolution factor in these systems. The results suggest that using nano additives, as well as the connecting plates, together is a promising way to enhance the solidification rate by up to 29.9%.

Natural Convection Compound Heat Transfer Enhancement by Discrete Rings and the Chimney Effect in a Vertical Cylinder

Natural Convection Compound Heat Transfer Enhancement by Discrete Rings and the Chimney Effect in a Vertical Cylinder

Aerothermal performance of a rotating two-pass furrowed channel roughened by angled ribs

• A compound-HTE method with angled ribs and furrowed passages was devised for cooling of gas turbine blades. • Without rotation, Nu 0 were raised to 4.9–4.77 times D-B levels with f 0 between 33.15 and 35.92 times Blasius equation values. • With rotation, overall Nu ( f ) on leading and trailing walls were 1.03–2.0 (1.10–1.66) and 1.02–1.91 (1.12–1.74) times Nu 0 ( f 0 ). • Thermal-performance indexes of static (rotating) channels were 1.53–1.45 (2.39–1.48). The aerothermal performance of a novel compound heat transfer enhancement method is assessed by measuring the full-field endwall Nusselt number map and Fanning friction factors of a rotating two-pass furrowed channel roughened by angled ribs. The tested Reynolds numbers, rotation numbers, and buoyancy numbers lie in the respective ranges of 5000–15,000, 0–0.4, and 0–0.183. Under the combined effects of the ribs and furrowed passages, the endwall-average Nusselt numbers of the static channel are upraised to 4.9–4.77 times the Dittus–Boelter levels. With rotation, the worst cooling condition develops at a rotation number of 0.05 with the spotted heat-transfer impediments to 0.77 and 0.82 times the static channel references on the leading and trailing endwalls, respectively. By raising the rotation number from 0 to 0.4, the Nusselt numbers on the stable endwalls are initially reduced from the static-channel levels and then converted to increase with the rotation number; however, the friction factors increase continuously. Over the rotating unstable endwalls, both the Nusselt numbers and the friction factors increase along with rotation number. When the buoyancy number is increased, the local Nusselt numbers on the leading and trailing endwalls increase with a negligible impact on their distribution patterns. The thermal-performance indexes of the rotating channel are reduced by increasing Reynolds number. To aid the relevant applications, two sets of experimental correlations are devised to determine the regional average Nusselt number and Fanning friction factor of the rotating two-pass furrowed channel fitted with angled ribs.

A comprehensive review on compound heat transfer enhancement using passive techniques in a heat exchanger

The heat transfer is a very interesting field for efficient, useful, eco-friendly reasons. The economic & environmental factors demand that we get the maximum possible efficiencies from thermal power plants possible only by making more efficient heat exchangers. Increasing the effectiveness of new and existing heat exchangers is a typical criterion of engineers involved with the design of heat exchangers. Augmenting process plant efficiency and finding new ways to reduce energy requirements is always an important design priority. Therefore, the heat transfer augmentation plays a substantial role in improving energy efficiency and development of high-performance thermal systems. The heat transfer augmentation techniques are classified as passive, active and compound techniques. The current article presents an extensive review of experimental and numerical studies on passive compound heat transfer augmentation techniques (PCHTAT), combining two or more of the current techniques can be employed all together to produce an enhancement larger than that produced by using only one augmentation. The compound technique arrangement are (i.e. helical-ribbed tube with double twisted tape inserts, Dimpled tube fitted with a twisted tape swirl generator, non-uniform wire coil, and twisted tape inserts. etc.) which covers the laminar and turbulent flow regions. Interactions between different enhancement methods will increase the overall heat transfer coefficient of the thermal systems than that produced by using only one augmentation technique.

Compound heat transfer enhancement of helical channel with corrugated wall structure

Newly, the use of corrugated structures in heat exchange devices has become a useful technique due to the combined merits of extended surfaces and turbulators. However, employing of this technique in curved channels is very scarce. To this aim, as a new passive compound technique, the periodical corrugated structure is proposed to enhance the overall hydrothermal performance of a helical channel with square cross-section. Firstly, calculated results for the smooth model are compared with existing empirical correlations in the literature to explore the reliability and accuracy of the current analysis. Secondly, the corrugated structure on each side (upper, lower, inner, and outer) as well as opposite sides (upper-lower and inner-outer) of the helical channel is tested individually. It is found that corrugating the side walls of the helical channel prevents the development of thermal boundary layers through the flow direction and changes continuously the location and strength of generated secondary flows and velocity contours, leading to more uniform temperature. Thirdly, three different levels of corrugation-amplitude (A = 0.8, 1.6, and 2.4 mm) and coil-diameter (D = 50, 100, and 150 mm) are considered. At the studied range of Reynolds number (100 ≤ Re ≤ 500), the best enhanced model is the case of upper-lower at A = 2.4 mm and D = 100 mm, and the maximum performance index of 1.46 is recorded for water flow through this model at Re = 400.

Experimental studies on the compound thermo-hydraulic characteristics in a 3D inner finned tube with porous copper fiber inserts

A novel easy-to-manufacture porous copper fiber sintered cylinder insert (PCFSCI) is designed to enhance the convective heat transfer performance in a three-dimensional inner finned tube (3D-IFT). The compound heat transfer enhancement of the PCFSCI and 3D inner fins is investigated at the Reynolds number ranging from 3921 to 14,290. The results show that the combination of PCFSCI and 3D-IFT creates a synergistic effect and a maximum performance evaluation criterion (PEC) value of 2.29. Moreover, the effect of porosity, diameter and spacing of the PCFSCI on heat transfer performance is investigated. The results show that the Nusselt number and friction factor increase with the decrease of porosity. The PCFSCI with a diameter of 8 mm presents a better PEC than PCFSCI with diameters of 12 mm and 10 mm. The spacing has a slight effect on the PEC, but a great influence on the Nusselt number and friction factor. These phenomena can be attributed to the fluid mixing between the core flow and the flow in the annular region, as well as the local vortexes and the changing of velocity gradient produced in the annular region.

Compound Heat Transfer Enhancement of Wavy Fin-and-Tube Heat Exchangers through Boundary Layer Restarting and Swirled Flow

This study performs a 3D turbulent flow numerical simulation to improve heat transfer characteristics of wavy fin-and-tube heat exchangers. A compound design encompassing louver, flat, and vortex generator onto wavy fins can significantly enhance the heat transfer performance of wavy fin-and-tube heat exchangers. Replacement of wavy fins around tubes with flat fins is not effective as far as the reduction of thermal resistance is concerned, although an appreciable pressure drop reduction can be achieved. Adding two louvers with a width of 8 mm to the flat portion can reduce thermal resistance up to 6% in comparison with the reference wavy fin. Increasing the louver number and width can further decrease the thermal resistance. Also, it is found that the optimum louver angle is equal to the wavy angle for offering the lowest thermal resistance. Therefore, compound geometry with three louvers, a width of 12 mm, and the louver angle being equal to wavy angle with waffle height to be the same as fin pitch of the reference wavy fin has the most reduction in thermal resistance of 16% for a pumping power of 0.001 W. Adding punching longitudinal vortex generators on this compound geometry can further decrease thermal resistance up to 18%.

Experimental thermal and flow characteristics in a traverse corrugated tube fitted with regularly spaced modified wire coils

The work presents experimental investigations on turbulent thermal and flow characteristics in a traverse corrugated tube (TCT) fitted with regularly spaced wire coils with gradually varying width (WCs-GVW). The experiments were performed in the TCT with WCs-GVW having constant heat flux conditions at Re = 6000 to 18000 by using air as working fluid. The goal motivated by this compound heat transfer enhancement technology was to induce transverse vortices by ribs and to generate swirl flows by WCs, aimed to impart flow separation and reattachment and flow mixing, and hence improvement in heat transfer. Three arrangements including the diverging arrangement of WCs (DWC), the converging arrangement of WCs (CWC) and the diverging and converging arrangement of WCs (DCWC) and five space ratios at S/D = 0.60, 1.55, 2.97, 5.34 and 10.09 were considered. Effects of those parameters on Nusselt number (Nu), friction factor (f), performance evaluation criterion (PEC), Bejan number (Be) and augmented entropy generation number (φs) were analyzed. The main findings are that the CWC arrangement possesses the lowest PEC and the Nu and f in the DWC arrangement increase with decreasing the space ratio. In contrast to the plain tube, the Nu in the TCT with WCs-GVW increases nearly 1.74 to 2.26 times while the f is in a range of about 4.18–10.68 times. The obtained results show that the maximum PEC of about 1.13 is achieved by the WCs-GVW with DWC arrangement at S/D = 10.09 and Re = 14102. However, all compound ones exhibit lower PEC values than the alone use of the TCT. In addition, it is found that the Be in the baseline of the plain tube is decreased via the employment of the alone TCT or co-use of the TCT and WCs-GVW, nevertheless, the total entropy production is dominated by the entropy production due to heat transfer under considered parameters. The φs in the plain tube is reduced to be less than unity by using the enhanced devices indicates the thermodynamic advantage of those TCT and TCT/WCs-GVW, and the lowest φs of about 0.5 is gained by utilizing the WCs-GVW with DWC arrangement at S/D = 0.60 and Re = 6517. At last, the empirical formulas of the investigated TCT and TCT/WCs-GVW in terms of Nu and f are proposed.

Numerical Analysis of Compound heat transfer enhancement by single and two-phase models in parabolic through solar receiver.

A three dimensional numerical investigation of turbulent forced convection of Alumina/dowtherm-A nanofluid inside a non-uniformly heated parabolic trough solar collector receiver equipped by two longitudinal fins for improving heat transfer by single and two-phase modeling has been carried out. The heat flux around the absorber tube was obtained applying Monte Carlo ray trace technique. The numerical results were validating with the empirical correlations existing in literatures and good agreement was obtained. The calculated results demonstrate that two-phase models predict a higher convective heat transfer coefficient however the predictions of Darcy friction factor by single and two-phase approaches are essentially the same. Two-phase mixture model has been used to investigate the influence of nanoparticles mean diameter on heat transfer phenomena; it found that a smaller nanoparticles increase better convective heat transfer.DOI: http://dx.doi.org/10.5755/j01.mech.23.1.14053

Thermal performance of compound heat transfer enhancement method using angled ribs and auxiliary fins

Detailed Nusselt number (Nu) distributions over two opposite endwalls of two parallelogram channels roughened by 45° ribs without/with in-line auxiliary fins are measured using steady-state infrared thermography method at 5000⩽Re⩽15,000. Pressure drop measurements are individually performed at isothermal conditions to detect the Fanning friction factors (f) for thermal performance factors (TPF) evaluations. Relative heat transfer and f augmentations for this type of passive compound heat transfer enhancement (HTE) method are assessed by comparing with the Nusselt numbers (Nu∞) and f factors (f∞) of smooth plain tube references at the same Reynolds number (Re). With present in-lined auxiliary fins on two opposite ribbed endwalls, the channel averaged HTE ratios fall between 4.43 and 4.21 with the corresponding f/f∞ ratios in the range of 6.5–3.35; giving rise to the TPF values between 2.37 and 2.81. As the comparative HTE ratios, f/f∞ values and TPF values for the similar ribbed channels without auxiliary fins are in the respective ranges of 3.93–3.61, 1.63–1.53 and 2.41–2.35, the present compound HTE method further elevates the Nu/Nu∞ ratios with the associated f amplifications but can still improves the TPF values in general. It is concluded that the present compound HTE method using angled ribs and auxiliary fins is applicable for improving thermal performances of turbulent channel flow.

Experimental Study on Compound Heat Transfer Enhancement of Transverse Corrugated Tube with Full Length Twisted Tapes

Experimental Study on Compound Heat Transfer Enhancement of Transverse Corrugated Tube with Full Length Twisted Tapes

Heat transfer enhancement of Al2O3-H2O nanofluids flowing through a micro heat sink with complex structure

The characteristic of flow and heat transfer of Al2O3-H2O nanofluids flowing through a micro heat sink with complex structure under constant heat flux is investigated experimentally. The volume fraction and Reynolds number of Al2O3-H2O nanofluids ranged from 0.1vol.% to 1.0vol.% and 100 to 700, respectively. Moreover, the effects of them on friction factor, Nusselt number, and thermal resistance are also considered. The results show that single phase model underestimates the friction factor and Nusselt number of nanofluids from the experiments. With increasing volume fraction and Reynolds number, both Nusselt number Nu and friction factor f of nanofluids increase while the average temperature at the bottom and thermal resistance decrease, as compared to deionized water. Moreover, new correlations of f and Nu are presented based on experimental data with the maximum deviation less than ±5%. The performance evaluation plot shows that the higher slope of values, i.e., higher volume fraction of nanofluids, gives better comprehensive performance of heat transfer. The compound heat transfer enhancement techniques, i.e., cavities in the micro heat sink and nanofluids can obviously enhance heat transfer.

Heat Transfer of Cu–Water Nanofluid in Parallel, Corrugated, and Strip Channels

Influences of corrugated-wavy and offset-strip channels as the core surface of plate-fin heat exchangers and copper nanoparticles as additives in deionized water were individually investigated. Then, the use of the corrugated-wavy and offset-strip channels along with the Cu–water nanofluid was examined as a compound heat transfer enhancement technique. The nanofluids were prepared by a one-step method, namely, electro-exploded wire technique, and the required properties of the nanofluids were systematically measured. It was found that the corrugated-wavy channel has a drastic effect on the performance of the heat exchanger, and the nanofluid with the lowest weight fraction (i.e., 0.1%) presents a better thermal-hydraulic performance. Moreover, the thermal-hydraulic specification associated with the application of the corrugated-wavy channel along with Cu–water nanofluid was discovered to be higher than those presented by the individual techniques. This compound technique can improve the considered performance factor by an average value of around 1.46 in the plate-fin heat exchanger.

Passive techniques for the enhancement of convective heat transfer in single phase duct flow

This review presents the main results of the experimental campaign on passive techniques for the enhancement of forced convective single phase heat transfer in ducts, performed in the last years at the Laboratory of the Industrial Engineering Department of the University of Parma by the Applied Physics research group. The research was mainly focused on two passive techniques, widely adopted for the thermal processing of medium and high viscosity fluids, based on wall corrugation and on wall curvature. The innovative compound heat transfer enhancement technique that couples together the effect of wall curvature and of wall corrugation has been investigated as well. The research has been mainly focused on understanding the causal relationship between the heat transfer surface modification and the convection enhancement phenomenon, by accounting the effect of the fluid Prandtl number. The pressure loss penalties were also evaluated. The principal results are presented and discussed.

Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO2–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO2–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.

Numerical predictions on fluid flow and heat transfer in U-shaped channel with the combination of ribs, dimples and protrusions under rotational effects

Recently, dimple and protrusion structure has been proved as an effective heat transfer augmentation approach on coolant channel due to its advantage on pressure penalty. A compound heat transfer enhancement technique, the combination of ribs, dimples or protrusions, is applied to a U-shaped square channel similar with the gas turbine blade internal passage. Considering the rotational condition of gas turbine blade on operation, the effect of rotation is also investigated for the coolant channel in order to approximate more to the real operation condition. Thus, the objective of this study is to discuss the effect of rotation on fluid flow and heat transfer performance of turbine blade similar U-shaped channel with the combination structure of ribs, dimples or protrusions. The investigated Reynolds number is 1.25 million and considered rotational number includes 0, 0.4 and 0.6. From the results, the fluid patterns of two-pass channel with compound heat transfer enhancement structure are presented for none-rotating and rotating cases. Meanwhile, spatially Nusselt distributions of roughened walls are obtained to reveal the heat transfer rates. Finally, the area averaged Nusselt number ratio and channel friction penalty are evaluated. The results indicate that rib-protrusion structure seems to be the most effective structure while rib-dimple structure has only slight advantage than ribbed channel. Furthermore, the additional friction penalty by dimple and protrusion structure is tiny. It can also be expected that, the thermal performance of this compound structure can be even improved after a denser arrangement of dimple/protrusion structure and optimal shape design.