Single-phase Heat Transfer Enhancement Research Articles

Morphological optimization of butterfly-shaped inserts for heat transfer enhancement in tubular heat exchangers: a numerical study

Single-phase heat transfer enhancement in heat exchangers represents a crucial issue for effective thermal treatment in many industrial applications, especially when highly viscous fluids or gases are processed. Increase of heat transfer capabilities might be obtained by passive techniques, such as tube corrugations or static inserts promoting boundary layers disruption at the fluid-wall interface. Different inserts have been proposed for tubular heat exchangers over the past decades, including twisted tapes, wired coils, conical rings and butterfly-shaped geometries. However, the literature lacks optimal geometries definitions. The aim of the present work is to achieve a morphological optimization of a butterfly insert through an adjoint approach to maximize heat transfer over pressure losses in heat exchangers tubes. In the present investigation, conjugate conduction-convection heat transfer in the computational domain is solved through a steady RANS approach. Turbulence is treated by means of the Realizable k-ε model with enhanced wall function. The model is successfully validated against experimental data. The optimization results suggest a reduction of the opening angle of the butterfly inserts of about 22°. The reported remarks can be used as guidelines for fabrication of optimized designs in tubular heat exchangers applications.

Experimental Investigations on Single-Phase Heat Transfer Enhancement in an Air-To-Water Heat Exchanger with Rectangular Perforated Flow Deflector Baffle Plate

Experimental analysis was conducted to investigate the turbulent heat transfer behaviors within a tubular heat exchanger, incorporating a novel baffle plate design. The new design includes a perforated circular baffle plate with a rectangular flow deflector that can be adjusted to different inclination angles. The baffle plate is strategically positioned at the entrance of the heat exchanger, resulting in a swirling flow downstream. To assess the impact of the baffle plate design, three baffle plates were placed longitudinally along the flow, with varying pitch ratios (l/D). The effects of pitch ratio (ranging from 0.6 to 1.2), deflector inclination angle (ranging between 30⁰ to 50⁰), and Reynolds numbers (ranging between 16000 to 29000) were examined. The outcomes highlighted the substantial impact of pitch ratio and inclination angle on the thermal enhancement factor. In particular, compared to single segmental baffle plates working under similar operating conditions. The result indicates that an inclination angle of 30° and a pitch ratio of 1 exhibited an average 41.49% augmentation in thermal-fluidic performance compared with an exchanger with a segmental baffle plate.

A novel convective heat transfer enhancement method based on precise control of Gyroid-type TPMS lattice structure

Innovative single-phase flow heat transfer enhancement technologies can improve heat dissipation efficiency and reduce the weight and volume of heat dissipation equipment. Triply Periodic Minimal Surfaces (TPMS) hold promise as a outstanding single-phase heat transfer enhancement structure. To further improve the convective heat transfer performance of TPMS, this study independently presents a novel lattice structure control method specifically designed for Gyroid-type TPMS, along with the corresponding mathematical equation. By adjusting the parameter “α” in the equation, the Gyroid lattice geometry can be controlled. This study named “α” as the “Lattice deformation control parameter of Gyroid”. Numerical simulations indicate that increasing α from 0 to 0.25 within the Reynolds number range of 282–2579 can lead to a 19.9–28.9% increase in the average convective heat transfer coefficient (h¯) of the Gyroid, a 0.3%-4.5% increase in the Nusselt number (Nu), and a 13.0%-20.3% decrease in the average friction factor (f¯). The research results suggest that optimizing α is a promising new technology for enhancing convective heat transfer of Gyroid-type TPMS, as it not only improves its heat transfer performance but also reduces its flow resistance. The main reason for the convective heat transfer enhancement of Gyroid-type TPMS with increasing α is the increase in the heat transfer area, while the reduction of flow resistance is mainly due to the formation of the “flow-guiding structure” inside the Gyroid.

Numerical Analysis of Turbulent Flow and Heat Transfer in Internally Finned Tubes

In this study, single-phase heat transfer enhancement in internally finned tubes is investigated numerically. The influence of fin number, helix angle, fin height, fin width and shape on the flow and heat transfer characteristics is studied. The research results indicate that the resistance coefficient and Nusselt number both increase with the increment of these parameters, among which the helix angle has the largest impact on the heat transfer enhancement. In addition, the shape of fins also has a small effect on the flow and heat transfer, and the heat transfer effect of triangular fins is the best.

Improvements of COBRA-TF on the effect of flow blockage during a LB LOCA with consideration of fuel relocation phenomenon

Coolability of the partially blocked core in a large break loss-of-coolant accident (LB LOCA) is one of the most important thermal-hydraulic concerns for nuclear reactor safety analysis. During blowdown phase and early stage of reflooding phase in the LB LOCA, the prevailing wall-to-vapor convective heat transfer plays an important role on the decay heat removal process. Experiments on single-phase convective heat transfer to vapor were conducted in 5×5 heater rod bundles containing 3×3 ballooned rods of 90% flow blockage ratio with consideration of fuel relocation phenomenon. The obtained experimental data were used to assess the single-phase heat transfer enhancement models of the COBRA-TF code. The assessment results showed underprediction of local heat transfer downstream of the flow blockage. Therefore, a new correlation has been proposed to improve the prediction capability of the conventional models by more accurately describing the flow blockage effect. The new correlation predicted the local heat transfer satisfactorily within a 20% discrepancy of the experimental data for various kinds of flow blockage configurations.

Subcooled flow boiling in a microchannel with a pin fin and a liquid jet in crossflow

This experimental study presents subcooled flow boiling of an engineering fluid — HFE-7000 — downstream a single pin fin in a microchannel. A liquid secondary jet was introduced into the flow to examine its merits pertinent to heat transfer enhancement. It was found that for HFE-7000 high wall superheats (ΔT(sat,ONB)∼40°C) were required for the onset of nucleate boiling (ONB). Once boiling started, nucleate boiling dominated. Heat transfer coefficient increased monotonically with heat flux, independent of mass flux and jet injections. Secondary flow injection, which was previously found to be an affective single phase heat transfer enhancement technique, showed limited potential for fully developed nucleate boiling.

Experimental investigations on single-phase heat transfer enhancement with longitudinal vortices in narrow rectangular channel

Four pairs of rectangular block as longitudinal vortex generators (LVG) were mounted periodically in a narrow rectangular channel to investigate fluid flow and convective heat transfer respectively in the narrow rectangular channel with LVG and without LVG. Both the channels have the same narrow gap ( d) = 3 mm, the same hydraulic diameter ( D h) = 5.58 mm and the same length to diameter ratio ( L/ D h) = 80.65. The experiments were performed with the channels oriented uprightly and uniform heat fluxes applied at the one side of the heating plate and single-phase water was used as test fluid. The parameters that were varied during the experiments included the mass flow rate, inlet liquid temperature, system pressure, and heat flux. In each of the experiments conducted, the temperature of both the liquid and the wall was measured at various locations along the flow direction. Based on the measured temperatures and the overall energy balance across the test section, the heat transfer coefficients for single-phase forced convection have been calculated. At the same time, in these experiments, the single-phase pressure drop across the channels was also measured. The correlations have been developed for mean Nusselt numbers and friction factors. Additionally, the visual experiments of infrared thermo-image recording the temperature on the outer wall of the heating plate have been conducted for validating the effects of LV. In these experimental investigations, both laminar regime and turbulent regime were under the thermo-hydraulic developing conditions, laminar-to-turbulent transition occurred in advance with the help of LV when Reynolds numbers vary between 310 and 4220. In laminar regime, LV causes heat transfer enhancement of about 100.9% and flow resistance increase of only 11.4%. And in turbulent regime, LV causes heat transfer enhancement of above 87.1% and flow resistance increase of 100.3%. As a result, LV can obviously enhance heat transfer of single-phase water, and increase flow resistance mildly.

Single-phase heat transfer enhancement in a curved, rectangular channel subjected to concave heating

Experiments were performed to ascertain the single-phase heat transfer enhancement provided by streamwise curvature. Curved and straight rectangular flow channels were fabricated with identical 5.0×2.5 mm cross-sections and 101.6 mm heated lengths in which heat was applied to a 32.3 mm radius concave wall in the curved channel and a side wall in the straight. Reynolds number ranged from 9000 to 130 000 and centripetal acceleration for the curved flow reached 315 times the earths gravitational acceleration. Nusselt numbers defined with hydraulic and thermal diameters were consistently underpredicted by previous correlations developed for full-periphery-heated channels but were accurately predicted when defined with heated width. Convection coefficients were enhanced due to flow curvature for all conditions tested, and detailed experimental correlations are provided for both the straight and curved configurations. Increasing Reynolds number produced different enhancement trends for different locations along the heated wall, decreasing the enhancement near the inlet and increasing it elsewhere downstream. Mechanisms responsible for the curvature enhancement are believed to be Dean vortices and a shift in the maximum axial velocity toward the concave wall. These mechanisms require a finite distance to develop sufficient strength to influence heat transfer, which explains the different enhancement trends observed for different locations along the heated wall.