Wire Coil Inserts Research Articles

An empirical study on evaporation heat transfer characteristics and flow pattern visualization in tubes with coiled wire inserts

In the present work, heat transfer and flow pattern visualization during forced convection evaporation of hydrocarbon R-600a (Isobutane) inside a horizontal tube with coiled wire inserts has been experimentally studied. The system consists of a pump, two preheaters, the test section, two condensers, flow meter and bypass. The test evaporator was an electrically heated copper tube of 1000mm length, 8.1mm inside diameter and a wall thickness of 0.71mm. Coiled wires with three different wire diameters of 0.5, 1.0 and 1.5mm and three different coil pitches of 10, 20 and 30mm were made and used in full length of test evaporator. The test runs were carried out at refrigerant mass fluxes from 109.2 to 505kg/m2s and heat fluxes between 18.6 and 26.1kW/m2. The use of coiled wires was found to increase the flow boiling heat transfer coefficients up to maximum value of 124% above the plain tube values with wire coiled having a twist ratio (PDin) of 1.243 and a thickness ratio (eDin) of 0.183. Four different flow patterns were observed and classified as bubbly, stratified, intermittent and annular flow. This study reveals the developments of the flow patterns. Also it was observed that the presence of coiled wire inserts induces disturbance into gas and liquid flows and so that the shape and motion of bubbles and slug are different from those observed in a plain tube. The correlation suggested by Shah [20] gave the best prediction to the flow boiling heat transfer coefficient in the plain tube within the studied ranges.

Local entropy generation rate through convective heat transfer in tubes with wire coil inserts

Purpose – The purpose of this paper is to present a numerical analysis of the flow and heat transfer in a tube with a wire coil insert. A second law analysis of the results is accounted for, in order to assess the local and overall entropy generation in relation with the increased pressure drop and convective heat transfer. A wire coil with p/D=1.25 and e/D=0.076 is selected as insert device. A Reynolds number range between 100 and 1,000 is investigated, which corresponds to the typical operating regimes in the risers of liquid solar collectors. Different wall heat fluxes and inclination angles allow to analyze the potential impact of mixed convection in the presence of tube inserts. Design/methodology/approach – Three-dimensional numerical simulations are performed using a finite-volume solver, assuming laminar flow conditions. Pure water and a mixture of water and propylene-glycol (20 percent) are used as working fluids, with temperature-dependent properties. Fanning friction factor, Nusselt number and local entropy generation results are obtained in the fully developed region. Findings – The friction factor results are successfully compared with a well-known experimental correlation for wire coil inserts. The earlier onset of transition is devised at Re > 300. Nusselt number augmentations between 2.5- and 6-fold are reported with respect to the smooth tube. The mixed convection regime encountered in the smooth tube for the operating conditions investigated is canceled in the wire coiled tube, owing to the opposed effect of the swirl flow induced and the bouyancy forces. Frictional, heat transfer and overall entropy generation rates are computed locally in the fully developed region, allowing to relate these results with the flow structures in the mixed convection smooth tube and in the wire coiled tube. A threefold decrease in the entropy generation rate is reported for tubes with wire coil inserts. Originality/value – An holistic understanding of the heat transfer enhancement in tubes with wire coil inserts is provided through the analysis of the flow pattern, Fanning friction factor, Nusselt number and local entropy generation rates. The reduced entropy generation in the enhanced tube serves as a performance criteria to confirm the positive effect of wire coil inserts in heat transfer for the operating regime under investigation, in spite of the increased pressure drop.

Tube-side heat transfer enhancement in flat-plate liquid solar collectors with wire coil inserts

This work presents an experimental study of the thermal-hydraulic characteristics in a flat-plate solar collector with two different geometries of helical coil inserts. Isothermal pressure drop tests are conducted in a horizontal tube using water as working fluid to obtain the fully developed Fanning friction factor for a range of Reynolds number Re = 80–8,000. Heat transfer tests are performed in the solar collector loop using a mixture of propylene-glycol and water at 30 wt%, covering the ranges Pr = 16–28 and Re = 140–600. A significant decrease in the temperature of the absorber is observed when the appropriate wire coil is selected for the given flow conditions, proving the beneficial effect of this type of insert device for the augmentation of the thermal efficiency in flat-plate solar collectors.

Experimental and numerical heat transfer investigation in turbulent square-duct flow through oblique horseshoe baffles

An experimental and numerical work has been carried out to study the heat transfer enhancement in a heat exchanger square-duct fitted with 30° oblique horseshoe baffles (HB). In the current work, air is passed through the HB-inserted duct having a constant surface heat-flux. The air flow and heat transfer behaviors are presented for turbulent flow region, Reynolds number ranging from 4000 to 25,000. The pertinent parameters of the 30° HB elements include three relative baffle-pitches (PR=P/H=0.5, 1 and 2) and five relative baffle heights (BR=b/H=0.05, 0.1, 0.15, 0.2 and 0.25). Influences of those parameters on heat transfer and energy loss due to friction in terms of Nusselt number and friction factor, respectively are studied. The experimental result shows that at a given BR, the smallest pitch spacing (PR=0.5) provides the highest heat transfer and friction factor. The HB at BR=0.25 and PR=0.5 yields the highest heat transfer and friction factor but the one at BR=0.2 and PR=1 gives the maximum thermal performance. In addition, the thermal performance of using the HB is much higher than that of the wire coil insert, in comparison with other turbulators. To understand the heat transfer mechanism, a numerical inserted-duct flow simulation is also conducted and the obtained numerical results are in good agreement with measurements. Numerical flow and heat transfer behaviors such as streamlines, temperature and Nusselt number contours of the duct flow model are also reported.

Heat exchanger network retrofit throughout overall heat transfer coefficient by using genetic algorithm

Heat integration through energy recovery is utilized to reduce energy consumption. Energy or heat recovery can be performed using heat exchanger (HE) in heat exchanger network (HEN). HEN is an arrangement of several interconnected HE, which is used to conduct heat recovery. This arrangement increases complexity of heat integration. In the existing HEN, some of the energy is wasted due to unproper HEN design. HEN retrofit can overcome this problem and increase the heat recovery in existing processes. In this research, HEN retrofit is performed by optimizing the maximum heat recovery (Q) without changing the area of heat transfer or adding new HE and the arrangement of HE in HEN. In order to fid out the maximum Q, genetic algorithm (GA) is used to search the best heat transfer coefficient (U) value. In this paper, three cases of optimization scenarios are performed by some constraints considered on the HEN model. In the first optimization scenario, U is optimized without the given the maximum and minimum limits. While in the second optimization scenario, U has limitation at the minimum value, which is the value of U on the initial design data. And on the third case optimization scenario, U has limits due to availability of existing technology, which is the increase in the maximum of U using internal fins, twisted tape insert, coiled wire insert, and helical baffle. Heat recovery obtained in the first case scenario optimization results was at 13.21%, whereas the second case scenario optimization was at 9.14%, and the third case scenario optimization was at 3.60% with an internal fin technology limitations, 2.77% by limitations of twisted tape inserts technology, 7.69% with coiled wire insert technology, and 4.61% with helical baffles technology.

Experimental Study of Heat Transfer Enhancement in a Heated Tube Caused by Wire-Coil and Rings

This study investigates heat transfer characteristics and the pressure drop of a horizontal double pipe heat exchanger with wire coil inserts. The amplification of convection heat transfer coefficient in the heat exchanger reduces the weight, size and cost of heat exchanger. One way of augmenting the heat transfer is to disturb the boundary layer. When an object is placed in a boundary layer, it affects the flow structure and alters the velocity and thermal profiles. The change is affected by the formation of jets and wakes in the boundary layer as it alters modifies transfer and friction coefficients on the wall. This paper studies the characteristics of the heat transfer and the pressure drop of a double pipe horizontal tube heat exchanger with an inserted wire coil and rings. Wire coil acts as a swirl flow, which increases turbulence and roughness whereas rings increase heat transfer as a promoter of turbulence and roughness. The experimental data sets were extracted from wire coils and rings tested within a geometrical range with a pitch of (P/D=1, 2, 4) and wire diameter of (d/D=0.05, 0.07, 0.11). For wire coil with d/D=0.11, P/D =1 and Reynolds number of 10000, the overall enhancement efficiency amounted to 128%.

Thermal and friction characteristics of laminar flow through a circular duct having helical screw-tape with oblique teeth inserts and wire coil inserts

Helical screw-tape inserts and wire coil inserts both individually are enhancement techniques. Naturally, therefore, it is worthwhile to try their combinations as a potential enhancement technique. The experimental friction factor and Nusselt number data for laminar flow through a circular duct having helical screw-tape and wire coil inserts have been presented. Predictive friction factor and Nusselt number correlations have also been presented. The thermohydraulic performance has been evaluated. The major findings of this experimental investigation are that the helical screw-tape inserts in combination with wire coil inserts perform significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain value of the fin parameter.

Heat exchanger network synthesis incorporating enhanced heat transfer techniques

Appropriate integration of tube- and shell-side enhanced heat transfer (EHT) techniques is essential for better thermal performance, lower heat transfer area requirement and reduced utility consumption in a heat exchanger network (HEN). In this study, EHT techniques have been integrated into grassroot HEN design to improve heat transfer rates and energy recovery. Based on performance and cost indices, an optimization model is proposed that identifies suitable tube-side and shell-side EHT techniques for implementation in grassroot HEN. The tube-side enhancement techniques considered include twisted-tape inserts, coiled-wire inserts, internal fins, the combination of coiled-wire inserts and internal fins, and the combination of twisted-tape and internal fins; external fins, helical baffles, and the combination of external fins and helical baffles are considered for the shell-side. Two examples are presented to demonstrate the capabilities of the models. The results obtained show that the proposed model ensures optimum selection of enhancement techniques, and savings in capital investment and utility costs. The study concludes that integration of enhancement devices into grassroot heat exchanger networks achieves capital investment and utility cost savings.

Experimental investigation on heat transfer and fluid friction correlations for circular tubes with coiled-wire inserts

Heat transfer and pressure drop data for air flow and water flow in smooth tubes with coiled-wire inserts were measured. The wire diameter-to-tube inner diameter ratio (e/d) and coil pitch-to-tube inner diameter ratio (p/d) are in the ranges of 0.0725 to 0.134 and 1.304 to 2.319 respectively. It is found that the Nusselt number (Nu) increases with the e/d value, whereas it increases with a decrease of the p/d value. As air is the working fluid, the dependence of the heat transfer enhancement of the wire coil on the Reynolds number (Re) is minor; as water is the working fluid, the heat transfer enhancement considerably decreases with an increase of the Re value. Two heat transfer empirical equations, one for air and the other for water, are proposed. For both air and water, a common fluid friction empirical equation is established. To effectively and efficiently enhance the heat transfer, for air, the proposed e/d and p/d values of the wire coil are 0.101 and 2.319, respectively; for water, the proposed e/d and p/d values are 0.101 and 1.739, respectively.

Heat transfer characteristics and pressure drop of the concentric tube equipped with coiled wires for pulsating turbulent flow

The influence of pulsation with different amplitudes on the heat transfer rates in a double-pipe heat exchanger where a coiled wire inserts around the outer surface of the inner tube is investigated. The heat transfer and pressure drop characteristics were investigated for a coiled wire inserts with different pitch values in absence of pulsation. Pulsation frequencies that were produced by using a reciprocating device ranged from 140 to 260cycles per minute (2.3–4.3Hz). Five different displacement amplitudes were used where the stroke length of the reciprocating piston was varied from 60 to 185mm. Thermally insulated wires having circular cross section of 2mm diameter, forming a coil of different pitches (p=6, 12 and 20mm) are used as turbulators. The investigation is performed for pulsating turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for counter flows. The experiments are performed for Reynolds numbers ranging from 4000 to 12,000. The experimental results indicated that heat transfer rates are enhanced with the increase in the pitch of coiled wire. This was attributed to the combined effect of both pulsation and coiled wires compared with the smooth annulus case (no coiled wire). The highest enhancement ratio in the Nusselt number compared with smooth annulus with no pulsation reached a value of about 12.7 (for the pitch ratio 10, stroke ratio 4.2 and Re=3856 with pulsating frequency 230cycles/min) while the friction factor is about 8.7times at same conditions. An empirical correlation was obtained and expresses the Nusselt number for different study parameters in 13% deviation.

An experimental study on heat transfer and pressure drop of MWCNT–water nano-fluid inside horizontal coiled wire inserted tube

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of Nanofluid turbulent flow inside horizontal copper tube with different wire coil inserts under constant heat flux. The plain and coil inserted tubes were used as the test section's geometries and were heated by an electrical coil heater to produce constant heat fluxes. Nanofluids with different particle weight concentrations of 0.05%, 0.1%, and 0.2% were prepared by dispersion of functionalized multi-walled carbon nanotubes (MWCNTs) in distilled water and stabilized by means of an ultrasonic device. The effect of different parameters such as Reynolds number, nano-particle concentration, wire diameter and coil pitch on heat transfer coefficient and pressure drop are studied. Observations clearly show that for a specific nanoparticle concentration, increase in both heat transfer and pressure drop is obtained by inserting coil wires. In average, 85% increase in heat transfer coefficient and 475% penalty in pressure drop was observed at the highest Reynolds number inside the wire coil inserted tube with the highest wire diameter. Finally, two new correlations are introduced using the results of the experiments for predicting the Nusselt number and friction factor of the nanofluid flow inside coiled wires inserted tubes.

Heat Transfer and Pressure Drop Characteristics of Graphene Oxide/Water Nanofluid in a Circular Tube Fitted with Wire Coil Insert

In this work, heat transfer and pressure drop characteristics of graphene oxide/water nanofluid flow through a circular tube having a wire coil insert were studied. The required graphene oxide was synthesized via the Hummer method and characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (SRD), and scanning electron microscope (SEM) methods. Dispersing graphene oxide in the water, nanofluids with 0.02, 0.07, and 0.12% volume fraction were prepared. An experimental set-up was designed and made to investigate the heat transfer performance and pressure loss of nanofluids. All experiments were carried out in the constant heat flux at tube wall conditions. The volumetric flow rates of the nanofluid were adjusted at 6, 8, and 10 L/min. Thermal conductivity, specific heat, density, and viscosity as thermophysical properties of the nanofluid were calculated using graphene oxide and water properties at the average temperature via appropriate relations. These properties were applied to calculate the convective heat transfer coefficient, Nusselt number, and friction factors for each experiment. Finally, the constant and exponents of Duangthongsuk and Wongwises's correlations for Nusselt number and friction factor were corrected by experimental results. The achieved experimental data have shown good agreement with those predicted. The results have shown that 0.12 vol% of graphene oxide in the water can enhance convective heat transfer coefficient by about 77%. As a result, it can be concluded that the graphene oxide/water can be used in the heat transfer devices to achieve more efficiency.

Investigation of the effect of using tube inserts for the intensification of heat transfer

In this work, heat transfer in channels containing inserts of different shapes was investigated using computational fluid dynamics (CFD) modeling techniques taking a gaslight water heater as an example. Three types of devices inserted in the water heater tube (flow swirlers) were investigated: star-shaped, coiled wire, and classic ones in the form of twisted tapes. In the present study, the RNG k-ɛ turbulence model is used to model the turbulent flow regime. This numerical simulation has been performed over a Reynolds number range of 5800–18500. In the studied range of Reynolds number the maximum thermal performance factor was obtained by the starry inserts with A star/A inlet = 0.50. The results have exposed that also the use of all tube inserts leads to a considerable increase in heat transfer and pressure drop over the smooth tube. In addition, the results revealed that both heat transfer rate and friction factor in the tube equipped with starry insert were significantly higher than those in the tube fitted with the coiled wire inserts and classic twisted tape.

Experimental Investigation to Evaluate the Optimum Performance of Helical Coiled Tube with Insert and Nano-Fluid

In the present work an experimental investigation is carried out to evaluate the performance of helical coiled tube with the swirl flow device using Al2O3nanofluid.The effects of wire coil insert with different parameter on heat transfer and friction loss in the helical tube were examined with Dean number (De) ranging from 700 to 2000. The circular or square coil wire has different cross sections, insertedin the tube with different pitch. The wire coil with Al2O3 nanoparticles with a diameter of 80nm dispersed in distilled water with volume concentrations of (0.08,0.1, 0.2and 0.3 vol.% ) were used as the test fluid. The effects of Dean Number, volume concentration of suspended nanoparticles, and wire coil on heat characteristics were investigated. The results reveal that the use of tabulators leads to a considerable increase in heat transfer and friction loss over those of a smooth tube. The Nusselt number increases with increasing of Dean number and reduction in pitch of wire coil. The square type of wire coil provides slightly higher heat transfer than the circular under the same conditions. Results show that the optimum heat transfer is caused of P=15mm of wire coils. Adding nanoparticles to the base fluid causes a significant enhancement in heat transfer characteristics. The overall enhancement in heat transfer using two mechanisms simultaneously compared to using pure fluid within the smooth helical tube exceeds over 213.2% (180% spring enhancement +33.2%Al2O3). The optimum results were found to be P=15mm, φ=0. 3Al2O3 t=2mm square cross section and De=1889. Finally, empirical correlations are developed of predicting Nusselt number of the flow with and without nanofluid. Comparison between the present result in reference results show good agreement.

Heat transfer enhancement of laminar and transitional Newtonian and non-Newtonian flows in tubes with wire coil inserts

This work presents an experimental study on two different wire coils inserted in a smooth tube using both Newtonian and non-Newtonian fluids to characterize their thermohydraulic behavior in laminar and transitional flow. Dimensionless pitches of the wire coils were chosen as p/D=1 and 2, whereas dimensionless wire diameter was e/D=0.09 for both wire coils. Non-Newtonian tests considered different viscosity types with concentration of 1% of CMC (carboxyl-methyl-cellulose) solution in water at several temperatures; a wide range of flow conditions has been covered: Reynolds number from 10 to 1300 and Prandtl number from 150 to 1900. Newtonian test were carried out with propylene glycol as working fluid, covering a similar range of Reynolds number as the previously indicated for non-Newtonian fluids. Isothermal pressure drop tests and heat transfer experiments under uniform heat flux conditions were performed, and results were contrasted with own experimental data for the smooth tube and with well-know analytical solutions. At low Reynolds numbers, both wire coils behave as a smooth tube but accelerate transition to critical Reynolds numbers down to 500. Maximum augmentations of Fanning friction factor of 3.5 times and of 4.5 times of Nusselt number have been found with respect to the smooth tube.

Using a wire coil insert for biodiesel production enhancement in a microreactor

In the present work, the application of wire coil to promote mixing in a microreactor during continuous production of biodiesel was studied. For this aim, soybean oil as a feedstock and potassium hydroxide as a homogeneous catalyzed were used. The influences of the various parameters such as geometric and operational conditions on the performance of biodiesel production were experimentally examined. Response surface methodology (RSM) in conjunction with the Box–Behnken method was used to statistically analyze and optimize the biodiesel production process. The comparison between two types of reactors (with and without wire coil) shows a significant enhancement in mixing during transesterification. The impacts of different wire coil lengths and wire coil pitchs on methyl ester conversion were also investigated. A reaction yield of 99% at the residence time of 180s was obtained in the modified microreactor. However, the measured pressure drop show that the microreactor equipped with wire coil consumed more energy. Therefore, performance ratio was defined to evaluate energy efficiency and the results show the advantage of using the wire coil insert in lower feed flow rates.

Comparative study on thermal performance of twisted tape and wire coil inserts in turbulent flow using CuO/water nanofluid

Heat transfer and friction factor analysis of CuO/water nanofluid flowing through a tube under turbulent flow conditions and with twisted tape (TT) and wire coil (WC) inserts were presented in this paper. The experimental investigations were performed in the Reynolds number range from 4000 to 20,000, volume concentrations of 0.1% and 0.3%, twisted tape inserts of h/d=5 and 10 and wire coil inserts of p/d=1.97 and 2.95. The experimental results indicated that under same operating conditions and flow rates, heat transfer coefficient, friction factor and thermal performance factor associated with nanofluid in a tube with wire coil inserts are higher than those with the twisted tape inserts. The Nusselt number enhancement for 0.3% nanofluid in a tube without inserts is 17.62%, 0.3% nanofluid in a tube with TT-2 is 31.88% and 0.3% nanofluid in a tube with WC-2 is 44.45% at a Reynolds number of 20,000 compared to water. Whereas, the friction factor enhancement for 0.3% nanofluid in a tube without inserts is 1.149-times, 0.3% nanofluid in a tube with TT-2 is 1.179-times and 0.3% nanofluid in a tube with WC-2 is 1.198-times at a Reynolds number of 20,000 compared to water. The thermal performance factor of 0.3% nanofluid in tube with twisted tape and wire coil inserts are 1.24 and 1.36 compared against water data respectively.

Experimental study on the combined effects of inclination angle and insert devices on the performance of a flat-plate solar collector

We report an experimental study investigating the thermal performance of a flat-plate solar collector using insert devices, over a Reynolds number range 200–8000 and a Prandtl number range 5–8, using water as the working fluid. A variety of conventional and novel insert configurations were considered which include, twisted-tape inserts, wire coil inserts and wire mesh insert. The results clearly indicate the enhancement of the Nusselt number by all insert devices. Comparison of the best inserts from different insert families show that in the laminar flow regime, the mesh insert performed the best whereas, the concentric coils achieved the highest Nusselt number augmentation in the turbulent regime, relative to the smooth pipe with no inserts. From the practical aspects, concentric coils were recommended as the best insert among those tested, which shows an overall Nusselt number enhancement of 110% in the low Reynolds number range and 460% in the high Reynolds number range. The impact of collector inclination of the performance of these insert devices was also investigated. The results show that the channel inclination does not have a significant impact on the Nusselt number enhancement.

Effect of inserting coiled wires on pressure drop of R-404A condensation

In this study, an experimental investigation was carried out to determine the increase of pressure drop during convective condensation of R-404A vapor inside coiled wire inserted tubes. Experiments were performed for a plain tube and five coiled wire inserted tubes. Data were collected for four mass velocities. For each mass velocity, six different vapor qualities were considered. From the results, it is found that the coiled wire inserts increased the pressure drop up to 1200% compared to plain tube values. An empirical correlation is also developed to predict the pressure drop of coiled wire inserted tubes during R404-A condensation with accuracy of ±20%.

Experimental study of heat transfer enhancement in a flat-plate solar water collector with wire-coil inserts

This work presents an experimental study on the use of wire-coil inserts to enhance heat transfer in a typical flat-plate solar water heater with optical efficiency of 77% and heat loss coefficient of 5.76 W/m2 K at a rated mass flow rate of 0.04 kg/s.The testing setup was fully designed following the requirements of the European Standard EN 12975-2 and was capable of measuring the performance of a standard and an enhanced collector under the same operating conditions (mass flow rate and inlet fluid temperature) and weather conditions.In order to expand the experimental range, both solar collectors were tested at five different values of mass flow rate, from 0.011 to 0.047 kg/s. Thermal efficiency results are provided and the differences between both collectors' performance are explained in terms of the changes that the wire-coil inserts produce inside the risers. In addition, to quantify the overall enhancement that the inserts promote in the standard collector, the experimental results have been processed to obtain a performance evaluation criterion (R3m) that computes the increase in useful power collected and pumping power.Wire-coil inserts promoted an average increase in thermal efficiency from 14% to 31% and an increase in the useful power collected with no additional pressure losses of up to 8–12%, for the values of mass flow rates investigated. The inserts have shown a lower potential of enhancing heat transfer for increasing mass flow rates. According to the present work, wire-coil devices can be successfully inserted within the flow tubes in solar water collectors for increasing its thermal efficiency.