Natural convection heat transfer in two-dimensional region formed by constant heat flux horizontal flat tube concentrically located in cooled horizontal cylinder studied numerically. The model solved using the FLUENT CFD package. The numerical simulations covered a range of hydraulic radius ratio (5, 7.5, and 10) at orientation angles from (0o up to 90o). The results showed that the average Nusselt number increases with hydraulic radius ratio, orientation angles and Rayleigh number. As well as enhancement ratio for Nusselt number at orientation angle 90o and hydraulic radius ratio 7.5 equal 24.87%. Both the fluid flow and heat transfer characteristics for different cases are illustrated velocity vectors and temperature contours that obtained from the CFD code. The results for the average Nusselt numbers are compared with previous works and show good agreement.
Heat exchangers are considered essential parts in many industrial applications. The construction process for heat exchangers is completely complex because accurate measurements of the penalty of pressure-drop and the rate of heat transfer are needed. Designing a compact heat exchanger with a high heat transfer rate, while utilizing the least amount of pumping power, is the main design challenge. The most recent investigations (including experimental results, numerical models, and analytical solutions) in the field of circular tube heat exchangers in general, and twisted tapes and wire coils in particular, are covered in this review article, which has more than 90 references. The enhancement techniques in heat exchangers tubes can generally be separated into three groups: active, passive, and hybrid (compound) approaches. This article reviews the literature on advancements made in passive enhancement approaches, with a specific focus on two types of passive promoters that employ twisted tapes and wire coils. The main contribution of this research is to highlight the behavior and structure of fluid flow and the heat transfer features for the twisted tapes and the wire coils. It also explains how these passive promoters can be used in circular tube heat exchangers to improve hydrothermal performance. Where, the installation of wire coils and twisted tapes considerably alters the flow pattern and aids in the improvement of heat transfer. Where, comprehending the behavior of fluid flow is crucial and contributes to the enhancement of heat transfer. Twisted tapes are less effective in turbulent flow than wire coils because they obstruct the flow, which results in a significant pressure reduction. When it comes to turbulent flow, the thermohydraulic performance of twisted tapes is lower to that of wire coils.
In this paper, the hydraulic-thermal performance of a double-pipe heat exchanger equipped with 45°-helical ribs is numerically studied. The ribbed double-pipe heat exchanger is modelled using three heights (H = 0, 2.5, 3.75, 5 mm) of 45°-helical ribs. Two numbers (4-ribs and 8-ribs) of 45°-helical ribs are attached on the outer surface of the inner pipe of the counter-flow double-pipe heat exchanger and compared with a smooth double-pipe heat exchanger. Three-Dimensional computational fluid dynamics (CFD) model for a laminar forced annular flow is performed in order to study the characteristics of pressure drop and convective heat transfer. In addition, the influence of rib geometries and hydraulic flow behaviour on the thermal performance is system-atically considered in the evaluations. The annular cold flow is investigated with the range of Reynolds numbers from 100 to 1000, with three heights of ribs at the same width (W = 2 mm) and inclined angles of (θ = 45°).The results illustrate that the average Nusselt number and pressure drop increase with an in-creasing number of ribs, the height of ribs and Reynold number, while the friction factor decreas-es with increasing Reynolds numbers. The percentage of averaged Nusselt number enhancement for three rib heights (H = 2.5, 3.75 and 5 mm) at 4-ribs is (34%, 65% and 71%), respectively, While for 8-ribs the enhancement percentage is (48%, 87% and 133%) as compared with the smooth double-pipe heat exchanger at Re = 100. The best performance evaluation criteria of (PEC) at (8-ribs, and H = 5 mm) is 2.8 at Re = 750. The attached 45-helical ribs in the annulus path can generate kind of secondary flows, which enhance the fluid mixing operation between the hot surface of the annular gap and the cold fluid in the mid of the annulus, which lead to a high-temperature distribution. Increasing the height of 45°-helical ribs lead to an increase in the sur-face area subjecting to convective heat transfer.