Increase In Friction Factor Research Articles

Experimental investigation on effect of perforated double V-cut twisted tape on thermo-hydraulic characteristics of heat exchanger tube

ABSTRACT To enhance overall thermal performance factor of tubular heat exchanger, use of twisted tape has been very effective. Twisted tape creates turbulence in the flow which enhances rate of heat transfer and friction factor. Present experimental study aims to limit this penalty of increase in friction factor and achieve high thermal performance factor. For this, present article analyzes the effect of different design modifications in twisted tape on thermo-hydraulic performance of heat exchanger tube. Different inserts, i.e. Simple Twisted tape (STT), Double V-Cut Twisted Tape (DVCTT) and Perforated Double V-Cut Twisted Tape (PDVCTT), of varying twist ratio (TR) 3, 4 and 5 are considered for the investigation. Further design modification such as V-cut with width of cut (wc) 5 mm, depth of cut (dc) 4 mm, and perforation of 5 mm diameter is produced in twisted tape. Water is used as the working fluid for the study. Data pertaining to Nusselt number and friction factor is collected for the analysis in turbulent region where Reynolds number varied from 4000 to 16,000. Maximum enhancement in Nusselt number and friction factor is recorded 2.44 times and 3.71 times of plain tube respectively at Re = 4000 in case of TR = 3. Further PDVCTT leads to maximum decrement in Nusselt number and friction factor 3.83% and 27.35% when compared with DVCTT. Maximum thermal performance obtained is 1.71 in case of PDVCTT with TR of 3 at Re = 4000.

Comparison of Heat Transfer and Friction in Pipes With Various Internal Roughness

Abstract The heat transfer and friction factor of turbulent pipe flows with different internal roughness are experimentally investigated. Three types of roughness in forms of a mesh, hemispherical elements, and a coil are added to the interior of pipes with an inner diameter of 52.5 mm. The working fluid is air, and the Reynolds numbers vary from 20,000 to 90,000 in increments of 10,000. For investigating the heat transfer, the pipe wall is heated to 375 °C while the inlet air remains at the room temperature. The measurements show that the mesh-type roughness results in a maximum Nusselt number, Nu, increase of approximately 6%, the pipes with hemispherical roughness increased the Nu by a maximum amount of 30%, and the coil increased Nu by up to 60% compared with the smooth pipe. The maximum increase of friction factor is 40% for the pipes with mesh-type roughness, 30% for pipes with hemispherical roughness, and 67% for pipes with coil roughness. The experimental results indicate that adding hemispherical and coil roughness to the internal surface of the pipe can lead to a significant improvement in the rate of heat transfer while adding a mesh-type roughness can have marginal improvements and comes with a large frictional loss penalty. The analysis shows that the highest thermohydraulic performance is achieved using the hemispherical roughness elements.

Improving Thermo-Hydraulic Performance with Varying Concentrations of Alumina Nanofluids: A Numerical Investigation

In the current study, the investigation of heat transfer and fluid flow Characteristics of Pure water when pass through a double tube heat exchanger (DTHX). This investigation has been conducted across various Reynolds Number to gain insights into their performance also conducted a computational fluid dynamics (CFD) simulation using the ANSYS-FLUENT 22 R1 software. The study employed mathematical models and thermophysical properties of nanofluids and water, which were sourced from existing literature. The analysis focused on comparing pure water, 1% Al2O3/H2O nanofluids. The investigation considered various operating variable as Reynolds Number and temperature across the inner, and outer tubes. Specifically, the Reynolds Number of a range of 2500 to 5500 at 80°C, and 2500 at 15°C for the respective tubes. Key findings are that friction factor for pure water, 1% alumina nf, 2% alumina nf, and 3% alumina nf is increased by 4.61%,11.42%,15.06% and 16.21% as compared to Gnielinski correlation in existing literature at a Reynolds Number of 2500 and this increase in friction factor is 5.66%, 13.79%, 18.03% and 19.61% respectively at Reynolds number of 5500. Nusselt number (Nu) for pure water, 1% alumina nf, 2% alumina nf, and 3% alumina nf is increased by 24.92%, 50.04%, 59.90% and 64.31% as compared to Gnielinski correlation in existing literature at a Reynolds Number of 2500 and this increase is 10.84%, 28.68%, 35.31% and 41.55% respectively at Reynolds number of 5500. The heat transfer coefficients (hi) for pure water, 1% alumina nf, 2% alumina nf, and 3% alumina nf is increased by 3.17%, 7.29%, 8.49% and 8.94% as compared to Gnielinski correlation in existing literature at a Reynolds Number of 2500 and this increase is 8.04%, 18.49%, 21.54% and 22.64% respectively at Reynolds number of 5500.

Experimental investigation on cooling tower performance with Al<sub>2</sub>O<sub>3</sub>, ZnO and Ti<sub>2</sub>O<sub>3</sub> based nanofluids

<p>This study deals with an experimental investigation of the thermal performance of a prototype mechanical wet cooling tower with a counter flow arrangement. Different volume concentrations ranging from 0.18 to 0.50 vol.% of stable Al oxide (Al<sub>2</sub>O<sub>3</sub>), Zn oxide (ZnO), and Ti oxide (Ti<sub>2</sub>O<sub>3</sub>) nanoparticles of 80, 35, and 70 nm diameter were considered. Water was taken as a base fluid, and the experiment was carried out at 60, 70, and 80 ℃, respectively, in laboratory conditions. The study revealed that an increase in the volume concentration of the nanofluids increased the cooling range, cooling efficiency, convective heat transfer coefficient, tower characteristic called number of transfer unit (NTU), and effectiveness of the cooling tower compared with water at the same mass flow rate and inlet temperature. However, increasing the volume concentration increased the viscosity of the nanofluids, leading to an increase in friction factor. For instance, for 0.18% volume concentration of ZnO, at an inlet water temperature of 66.4 ℃ and water/air (L/G) flow ratio of 1.93, the cooling range increased by 3.62%, cooling efficiency increased by 33.3%, and NTU increased by 50.5% compared with fresh water (FW).</p>

Numerical Investigation on Flow and Heat Transfer characteristics of Pure Water in Concentric Triple Tube Heat Exchanger

In the current study, the investigation of heat transfer and fluid flow Characteristics of Pure water when pass through a triple tube heat exchanger (CTTHX). this investigation has been conducted across various Reynolds Number to gain insights into their performance also conducted a computational fluid dynamics (CFD) simulation using the ANSYS-FLUENT software. Result obtained was validated by comparing to empirical correlation data found in the existing literature. The investigation considered various operating variable as Reynolds Number and temperature across the inner, intermediate, and outer tubes. Specifically, the Reynolds Number of 2000 at 305 K, a range of 2000 to 4500 at 340 K, and 2000 at 295 K for the respective tubes. Key findings are that friction factor increase by 12.5% as compared to correlation in existing literature. And Nusselt number (Nu) increase by 50% in the range of Reynolds Number (Re) from 2000 to 4500.

THERMAL DIFFUSION AND RADIATION ABSORPTION IMPACTS ON CHEMICALLY REACTIVE AND RADIATIVE NANOFLUID FLOW PAST AN EXPONENTIALLY ACCELERATED VERTICAL POROUS PLATE

The role of thermal diffusion (Soret) and radiation-absorption influences on the unsteady MHD boundary layer flow for Silver (Ag) and Copper (Cu) water-based nanofluid over an accelerated exponentially vertical porous plate has been investigated. The varying temperature and concentration at the plate are also considered. The governing equations of the flow are numerically resolved to employ the finite-difference system of the implicit Crank-Nicolson method. Fluid velocity, f luid temperature, and species concentration were all plotted for meaningful physical parameters. For both nanofluids, non-dimensional metrics such as the friction factor, Nusselt number, and Sherwood number at the surface are tabulated. The data obtained from the literature provide strong validation for the findings. The f inding of the study is that both nanofluids’ velocities rise with increases in Soret and radiation absorption parameters. For increased values of Soret and radiation absorption parameters, the friction factor and Nusselt number increase. Sherwood numbers drop for the Soret parameter and rise for the radiation absorption.

Hierarchical Deployment of Vortex Generators for Wake Management in Finned-Tube Heat Exchanger

While developing thermal management systems, volumetric compactness of the heat exchange module is a critical design factor. This three-dimensional computational analysis seeks to boost the thermal compactness of fin-and-tube heat exchangers by employing winglet-type vortex generators. Due to the fact that the thermo-hydraulic performance of vortex generators in such thermal systems is determined by three distinct design parameters, namely the attack angle, spatial location, and geometric aspect ratio, a concurrent parametric analysis of all three design parameters is conducted to gain a thorough understanding. In addition, regression analysis is applied to develop functional correlations that can predict the enhanced thermo-hydraulic performance of the heat exchangers. The improvements in wake-affected heat transfer brought on by the modifications are also specifically explored, along with other aspects of the augmentation mechanism. The highest thermal augmentation over the wake-affected fin is seen as 120%. Based on a thermo-hydraulic tradeoff, the friction factor increases exponentially with the attack angle, and the rate of increase is lowest for 15–30 degree range of attack angle and highest for 45–60 degree range. Certainly, there are multiple locations where winglets can be erected, and heat transfer enhancement by each potential location incurs linearized increase in the friction factor.

Numerical analysis of thermohydraulic and exergetic performance in corrugated spiral tubes with diverse arc corrugation configurations

This study numerically analyzes the thermohydraulic performance and exergy destruction in a corrugated spiral tube, considering inward, outward, and combined inward-outward configurations versus smooth spiral tubes. It explores how key corrugation geometries like width, height, and pitch as well as flow parameter such as Dean number influence friction factor, Nusselt number, exergy efficiency, and thermal performance factor. The turbulence and thermal aspects of the flow physics are modeled by incorporating the pertinent governing equations, utilizing the SST k-ω turbulence model and Reynolds-Averaged Navier-Stokes (RANS) equations. The results demonstrated that introducing corrugated walls in various configurations enhances the Nusselt number within the spiral tube compared to its smooth counterpart. Nevertheless, this enhancement is accompanied by an increase in both friction factor and exergy destruction. Employing inward corrugations leads to a remarkable 53% surge in the Nusselt number when compared to the smooth tube. Elevating the corrugation height amplifies heat transfer and Nusselt numbers for all configurations, with the inward corrugation (IC) exhibiting the most substantial escalation. Additionally, a corrugation width of (w/Dh=0.1) engenders maximum thermal performance factor values across scenarios. Augmenting the corrugation pitch increases the Nusselt number due to greater heat transfer area, with the inward topology (IC) conferring maximal values. Although the Nusselt number rises with escalating Dean number, the thermal performance factor declines for all tubes owing to a disproportionately larger pressure drop increase. The outward corrugation tube (OC) sustains the highest performance factor across Dean numbers, while the inward-outward configuration (IOC) possesses the lowest.

Conceptual design of narrow annular channel heater with rectangular wire coil for the simulator of solid-fuel thorium molten salt reactor

To heat the simulator core of a solid-fuel thorium molten salt reactor, the narrow annular channel heater with rectangular wire coil separating from the tube wall is proposed. Three-dimensional numerical models are developed, and the simulated result shows that negative axial, radial and tangential velocities in the enhanced narrow annular channel heater are generated compared to the smooth narrow annular channel heater. As the wire width increases and helical pitch decreases, the Nusselt number and friction factor increase. The maximum Nusselt number for enhanced narrow annular channel heater is 5.6 times that for smooth narrow annular channel heater. The overall performance of enhanced narrow annular channel heater is evaluated at a constant pumping power, the value of which is in the range of 1.21–1.85. The best enhanced narrow annular channel heater for the simulator of solid-fuel thorium molten salt reactor is determined based on the overall performance. The maximum outer wall and fluid temperatures of enhanced narrow annular channel heater for the simulator of solid-fuel thorium molten salt reactor are 937.94 K and 931.01 K, which are within the safety limit values of 1143.15 K and 977.15 K, respectively. This design will provide a new method to heat molten salt.

Examining the effect of backward/forward-facing wavy channels on the thermohydraulic performance of a printed circuit heat exchanger under the laminar flow regime

The printed circuit heat exchanger (PCHE) has been used in many important engineering applications because of its advantages, including its small size and high efficiency. This study aims to enhance the thermohydraulic characteristics of the PCHE by using new channels characterised by a three-dimensional flow path. These proposed backward/forward-facing wavy channels (BFFWCs) are a combination of a wave path in the horizontal plane and a U-shape path in the vertical plane. Under laminar water flow on both sides of the PCHE, a numerical study was conducted to assess the thermohydraulic characteristics of the proposed PCHEs with various wave amplitudes for channels and angles of attack for six models of BFFWCs on the basis of the conventional PCHE. The evaluation parameters that were adopted are the Nusselt number, friction factor, thermohydraulic performance criterion, and PCHE's effectiveness. Two models of the PCHE were manufactured and tested—one for the straight channels and the other for the proposed channels—to verify the accuracy of the numerical simulation results by comparing them with the experimental outcomes. Results indicated that the three-dimensional flow path has a positive influence on enhancing the heat transfer rate of the exchanger, with a slight increase in the pressure loss. The Nusselt number and friction factor increase with increasing wave amplitude and angle of attack. Compared with conventional PCHE, the maximum improvement in the Nusselt number is as much as 71 % and 78 % for model 3 (amplitude = 0.8 and angle of attack = 30°) and model 6 (amplitude = 0.8 and angle of attack = 60°). The thermohydraulic performance of model 3 was better than that of model 6. The maximum obtained thermohydraulic performance criterion reaches about 1.43 and 1.40 for models 3 and 6, respectively.

Compound passive heat transfer augmentation techniques: A comprehensive review

AbstractThe function of energy is becoming increasingly vital in meeting the requirements of modern societies and sustaining rapid economic and industrial growth globally. Heat transfer equipment has been employed for heat recovery and conversion in various domestic and industrial applications. Therefore, due to the recent global energy crisis, boosting the thermal efficiency of energy systems has become an essential requirement, which would reduce both their size and rates of energy demand. There are active and passive methods for boosting heat transfer rates. As they have no moving components, passive methods are more affordable and dependable than active ones. Applying two or more passive techniques concurrently will result in a higher heat transmission rate than any approach working independently. The current article comprehensively reviews experimental and computational investigations of passive compound forced convection heat transfer augmentation techniques at laminar, transition, and turbulent flow regimes. This article focuses on compound techniques arrangement in the form of turbulators, typical twisted tapes (TTs), surface roughness, vortex generators, and so on. The pioneering research suggests that using a lower twist ratio, lower pitch, and smaller winglet angles in TTs can result in higher heat transfer rates, albeit with a slightly increased friction factor. The combination of alternate‐axes and wings in TTs leads to more effective heat transfer enhancement within the tube.

Comparison study on thermo-hydraulic performance of circular and airfoil shaped wire-wrapped rod bundle

The turbulent flow behavior and heat transfer performance of wire-wrapped rod bundles have a significant impact on the thermal–hydraulic characteristics and safety of Generation IV nuclear reactors. In this study, the airfoil-shaped wire-wrapped bundle is proposed to improve the comprehensive performance of advanced reactor cores, and the thermal–hydraulic performance has been compared with the circular-shaped wire-wrapped bundle. Special attention has been paid to the temperature variation and friction pressure loss caused by the wire wraps. Specifically, the generation of the secondary flow is the main factor. The airfoil-shaped wire-wrapped bundle exhibits a half friction pressure loss, a more uniformed temperature distribution, and a reduction of the hot spot temperature compared to the circular-shaped wire-wrapped bundle. Moreover, the airfoil-shaped wire shows a 16.3% increase in Nusselt number and a 2.0% increase in friction factor compared to circular-shaped wire, resulting in an 8.7% increase in performance evaluation criteria (PEC). The airfoil-shaped wire-wrapped bundle demonstrates better comprehensive performance than the circular-shaped wire-wrapped bundle in terms of PEC.

Effect of diameter, twist angle, and blade count on the thermal-hydraulic performance of a decaying twisted swirler

Inserting a decaying swirler into a heat exchanger has been shown to improve heat transfer with minimal effect on the friction factor. The study analyses the effect of diameter, twist angle, and blade count on the thermal-hydraulic performance of a Decaying Twisted Swirler (DTS) in a horizontally heated tube. The diameter, twist angle, and DTS's blade count are examined for 13.5 mm–15.5 mm with a 0.5 mm interval, 0°–360° with a 60° gap, and 2 to 6 blades, respectively. The Nusselt number, friction factor, and thermal-hydraulic performance are examined for Reynolds numbers between 4583 and 35000. The relative Nusselt number and friction factor increase as DTS diameter and twist angle increase, reaching a maximum value at Re = 4583. Despite this, the relative Nusselt number dispersed as the blade count increased. The relative friction factor increases as the blade count increases. Maximum relative Nusselt number and friction factor reached 1.64 and 3.25, respectively with DTS's 15.5 mm diameter, 360° angle, and 4 blades. Nonetheless, the thermal-hydraulic performance is greatest when the DTS has a diameter of 15.5 mm, a twist angle of 180°, and 2 blades with 1.17.

Effect of an inclined magnetic field on unsteady mixed convective stagnation point flow over a permeable stretching sheet with radiative heat transfer

In this study, we analyzed the time-dependent, 2D, inclined magnetohydrodynamics mixed convection stagnating point flow of nanoparticle toward a permeable stretchable surface with radiative heat effect. Four different types of water-based nanomaterials, namely titanium dioxide copper aluminum oxide and copper oxide are analyzed in this investigation. Numerous engineering disciplines, including energy systems, material processing, thermal management, and environmental engineering, can benefit greatly from research in mixed convection, MHD, and nanofluids. The boundary layer representations of the dominating partial differential equations PDEs are converted into strong nonlinear ordinary differential equations ODEs utilizing similarities approach. The ODEs are computed numerically employing 4th order Runge–Kutta methodology-based shooting scheme. Few special situations, a remarkable alignment is determined between the present study and the outcomes obtained in the existing research. The impact of numerous pertinent variables on the prominent quantities such as velocity, temperature and concentration distributions, drag friction coefficient and heat transport are demonstrated graphically. It is noticed that the velocity and thermal curve decrease as the nanoparticle volume friction factor increases. Moreover, it is analyzed that higher values of nanoparticle concentration diminishes the velocity and temperature distributions.

Two-tube heat exchanger with variable groove angle on the inner pipe surface: Experimental study

Flow patterns in fluids affected by a grooved surface tube are different from a smooth pipe. The importance of a compact and optimal design of heat exchangers has led to extensive studies in the past focused on geometry (circular, square, etc.), depth, and distance between grooves. In terms of novelty, the present work studies the influence of the groove angle of the inner tube surface on fluid flow and thermal performance. The study method was experimental, and Wilson plots were used to determine the heat transfer coefficient. Three test sections, including smooth pipe and helical grooves, were made. Experiments were performed for the base case (smooth pipe) within the Reynolds number range of 9000–18000 and a constant inlet temperature of 20 °C for cold water. The hot water was kept constant in the Reynolds number range of 2700, and the inlet temperature was 50 °C. Heat transfer, friction factor, effectiveness, and the number of thermal units were evaluated as significant features in heat exchanger performance analysis. The results were presented in two arrangements of parallel and counter fluid flow for helical groove tubes, in comparison, with a smooth pipe. In low Reynolds, geometry and groove angle affect heat transfer more than flow rate. Heat transfer enhances as the groove angle becomes more vertical to the flow. Per 30° change in the slope of the spiral groove from parallel to perpendicular to the fluid flow, the Nusselt number and the friction factor increase by 25–32% and 8–9%, respectively.

Analysis on enhanced turbulent heat transfer and flow characteristic in a twisted and dimpled oval tube

This work reveals the heat transfer and flow characteristic in a twisted and oval tube under turbulent flow and constant heat flux conditions. Numerical analyses were carried out on several twisted and oval tube configurations. In order to clearly reveal the effect of dimples in the swirl flow, the twisted oval tubes with and without dimples are considered. In order to enhance the convective heat transfer, the dimples were placed in the narrowing region of the twisted tube where the heat transfer was inefficient. The results indicate that the dimples on the twisted tube significantly increase the heat transfer, although they cause to slightly increase pressure drop penalty. While the decrease in the pitch length induced to enhance the heat transfer, it significantly and negatively affected the hydraulic performance of the tube. Furthermore, while the dimples on the twisted oval tube induce the average Nusselt number to increase by 14.8%, they cause to the friction factor increase by 18.0%. The best configuration that is the case of dimpled twisted oval tube having pitch length of 200 mm at Reynolds number of 10,000, yields thermo-hydraulic performance criteria of 1.428.

Correlations of Heat Transfer and Fluid Flow Data for Lattice Brick Settings in Tunnel Kilns

This paper proposes correlation equations of heat transfer and pressure drop for the design and operation of tunnel kilns loaded with lattice brick settings of different geometrical parameters in the form of Nusselt number and friction factor. The developed correlation equations considered parameters that were not investigated in previous studies, such as the relative roughness of the bricks and the stack channels, and they also extended the Reynolds numbers to a practical range that was not covered before in a simple, practical form. The correlation equations are valid for Reynolds number between 125 and 10,200, Prandtl number between 0.68 and 0.73, brick’s relative roughness between 0.23 and 0.93, voidage fraction between 0.48 and 0.653, and the geometrical parameters of the tested lattice brick settings. The achieved correlations of the Nusselt number and the friction factor are well compared with the available correlations in the literature in their valid range of parameters. It is found that Nusselt numbers and the friction factors for low-density are higher than those of high-density settings for all considered parameters except the voidage fraction. The effect of the considered parameters confirms that Nusselt numbers increase and the friction factors decrease substantially with the Reynolds number and slightly with the Prandtl number. At a constant Reynolds number, both the Nusselt number and the friction factor increase as the brick’s relative roughness is increased. Moreover, as the stack channel spacing is increased, the Nusselt number decreases, and the friction factor increases. The voidage fraction of the setting has a monotonic effect on both Nusselt numbers and friction factors. Nusselt numbers for high-density are higher than those for low-density settings as the voidage fraction varies.

Experimental investigation of thermal performance of novel type vortex generator in rectangular channel

Vortex generators are objects with geometries specifically designed to increase heat transfer. In this study, the thermal performance of a novel vortex generator that can generate longitudinal vortices in a common flow-down pattern was experimentally investigated with Reynolds numbers in the range of 5000–25,000. The vortex generators were placed on the base surface of the channel in such a way that they were mirror images of each other according to the geometric symmetry plane in the main flow direction. At a fixed inclination angle, the effects of the ratio of the transverse distance between two vortex generators in a row to the channel width (transverse pitch ratio) and the ratio of the longitudinal distance between two vortex rows to the channel height (longitudinal pitch ratio) on heat transfer were analyzed. First, the experiments were performed for a single row by increasing the number of the vortex generators in the row (2, 4, 6, and 8) so that the transverse pitch ratios were 0.50, 0.25, 0.16, and 0.12. According to the results, as the transverse pitch ratio decreased, the Nusselt number and friction factor increased, so the highest increases were realized at the transverse pitch ratio of 0.12, reaching 1.44 and 2.07 times greater values compared with the smooth channel, respectively. However, the highest thermal enhancement factor was obtained at the transverse pitch ratio of 0.16. Then, vortex generators were placed with an inline arrangement keeping the transverse pitch ratio of 0.16 constant, and the experiments were executed for longitudinal pitch ratios of 1, 1.5, 3, and 5. The results revealed that the Nusselt number and friction factor increase with the decrease in longitudinal pitch ratio, but the highest thermal enhancement factor value (1.59) was obtained at the longitudinal pitch ratio of 1.5. In addition, the experiments were also performed using a staggered arrangement, but the inline arrangement proved to be more efficient than the staggered arrangement in terms of thermal performance. The maximal thermal enhancement factor was achieved for the inline vortex arrangement at the transverse pitch ratio of 0.16 and longitudinal pitch ratio of 1.5.

Numerical study of location and depth of rectangular grooves on the turbulent heat transfer performance and characteristics of CuO-water nanofluid flow

This current work expresses numerical simulation of forced turbulent flow convection in a grooved cylinder. Rectangular grooves with a spacing of A = 1, A = 1.1, and A = 1.3, and groove depth to cylinder diameter of e/D = 0.1 and 0.2 were considered. This research concentrates on the effect of groove depth, location of the grooves and CuO nanoparticles on the heat transfer for Reynolds numbers 10000, 12,500, 15,000 and 17,500 in volume fractions of 0, 1, 2, 3 and 4% of nanoparticles. Results show that grooves improve heat transfer. This behavior at a lower A ratio results in a significant Nu number increase so that the highest Nu number occurs for A ratio of 1, 1.1 and 1.3. Increasing e/D ratio, due to increasing the channel section in this area, results in loss of velocity and dissipation of flow momentum, resulting in lower convective heat transfer and lower Nu number. Changing the pitch for e/D = 0.1 results in a 1.1 to 1.6 times increase of Nu number compared with the smooth channel, and for e/D = 0.2 this value is 1.1–1.5 times the smooth channel for similar Re, φ and geometry. Changing groove pitch at e/D = 0.1 results in a 2.1–2.9 times increase in friction factor compared with the smooth channel in similar conditions. For e/D = 0.2, this increase is 1.8–2.8 times the smooth channel. In low Re, the thermal performance is higher than in higher velocities. This is because the grooved channel acts as a smooth channel at high Re, and the average Nu does not have significant growth.

Numerical simulation of flow and heat transfer of n-decane in sub-millimeter spiral tube at supercritical pressure

The flow and heat transfer characteristics of n-decane in the sub-millimeter spiral tube (SMST) at supercritical pressure (p = 3 MPa) are studied by the RNG k − ε numerical model in this paper. The effects of various Reynolds numbers (Re) and structural parameters pitch (s) and spiral diameter (D) are analyzed. Results indicate that the average Nusselt number (Nu¯) and friction factor (f¯) increase with an increase in Re, and decrease with an increase in D/d (tube diameter). In terms of the structural parameter s/d, it is found that as s/d increases, the Nu¯ and f¯ first increase, and then decrease. and the critical structural parameter is s/d = 4. Compared with the straight tube, the SMST can improve Nu¯ by 34.8% at best, while it can improve f¯ by 102.1% at most. In addition, a comprehensive heat transfer coefficient is applied to analyze the thermodynamic properties of SMST. With the optimal structural parameters of D/d = 6 and s/d = 4, the comprehensive heat transfer factor of supercritical pressure hydrocarbon fuel in the SMST can reach 1.074. At last, correlations of the average Nusselt number and friction factor are developed to predict the flow and heat transfer of n-decane at supercritical pressure.