Non-circular Passages Research Articles

Turbulent heat transfer and nanofluid flow in a triangular duct with vortex generators

Triangular conduits provide low pressure drop and low heat transfer coefficient compared to circular and other shapes of non-circular passages. The advantage of their low pressure drop motivated this research to increase the heat transfer rate by using two passive methods; vortex generators and nanofluids. Turbulent flow-forced convective heat transfer of Al2O3/water and CuO/water nanofluids in an equilateral triangular passage with a delta-winglet pair of vortex generator is numerically studied in three-dimensions. A two-phase mixture model is utilized to simulate the fluid flow considering variable properties for nanofluids whereas those of solid nanoparticles are constant. The study is carried out to examine the effect of the different types of nanofluids, different volume fractions with a wide range of Reynolds numbers on the heat transfer and pressure drop penalty. A comparison between the nanofluid and the base fluid heat transfer is performed in this research. The Al2O3/water nanofluid shows great enhancement in heat transfer with the maximum overall performance of 45.7% at 3vol.% and Re=16,000 compared to the base fluid. High heat transfer and low pressure drop are observed by using nanofluid and VG simultaneously.

Blood flow in non-circular microchannel under pulsating condition

With the advent of point-of-care diagnostics systems, investigation into properties of blood at micron scales is gaining fundamental importance. Past research has shown that blood displays significantly different properties at small scales than at conventional scales. This study investigates properties of blood flow in small non-circular passages (hydraulic diameter: 95-960 μm) under pulsatile condition. The observations are compared with flow of water under otherwise similar conditions. Prominent observation includes a more stable response to abrupt mass flow rate fluctuations as compared to water, which is attributed to the presence of deformable cells in blood. The study also reveals that, the pressure drop for blood flow with pulsations is less than for steady condition with the difference increasing with a reduction in microchannel size and flow rate. Such a comparative study facilitates development of models for blood flow at micro-scales, and will eventually aid in the design of future micro-Total ...

Shape design of internal cooling passages within a turbine blade

The article concerns the optimization of the shape and location of non-circular passages cooling the blade of a gas turbine. To model the shape, four Bezier curves which form a closed profile of the passage were used. In order to match the shape of the passage to the blade profile, a technique was put forward to copy and scale the profile fragments into the component, and build the outline of the passage on the basis of them. For so-defined cooling passages, optimization calculations were carried out with a view to finding their optimal shape and location in terms of the assumed objectives. The task was solved as a multi-objective problem with the use of the Pareto method, for a cooling system composed of four and five passages. The tool employed for the optimization was the evolutionary algorithm. The article presents the impact of the population on the task convergence, and discusses the impact of different optimization objectives on the Pareto optimal solutions obtained. Due to the problem of different impacts of individual objectives on the position of the solution front which was noticed during the calculations, a two-step optimization procedure was introduced. Also, comparative optimization calculations for the scalar objective function were carried out and set up against the non-dominated solutions obtained in the Pareto approach. The optimization process resulted in a configuration of the cooling system that allows a significant reduction in the temperature of the blade and its thermal stress.

An advanced nonlinear switched heat exchanger model for vapor compression cycles using the moving-boundary method

Simulation is commonly used to develop control and diagnostic algorithms. Because vapor compression cycles are essentially heat management devices, transient and steady-state heat exchanger performance must be predicted accurately. However, for hardware- and software-in-the-loop purposes, the simulation must run in real-time. To reconcile these competing needs, a new lumped parameter or moving-boundary heat exchanger model was created. Accuracy concerns are addressed by including finned surfaces, nonlinear air temperature distributions, and non-circular passages. In its current form, the model is applicable to single pass, cross-flow heat exchangers. The mathematical basis of the model is given and shown to be consistent with integral forms of the energy and continuity equations. Although known to be more computationally efficient than finite volume models, moving-boundary models become singular and fail under certain conditions. To address this shortcoming, particular attention was focused on algorithms for switching between different representations and rezoning wall temperatures. Robustness to changing flow regimes is demonstrated through simulation test cases, and model application to a chiller system is shown. As such, the model provides improved accuracy, robustness, and operating range while maintaining real-time capability.

LIQUID-CRYSTAL TRANSIENT TECHNIQUE FOR MEASUREMENT OF IN-TUBE LOCAL CONDENSING HEAT TRANSFER COEFFICIENTS

An experimental technique, using liquid crystals and the transient method, was developed to measure in-tube condensing local heat transfer coefficients. Measurements wilh R-l2 inside a horizontal, smooth circular tube is presented. This article describes the experimental procedures used to achieve a step change in the surface temperature with a special valve. The test section consists of a transparent glass tube obserued from outside. The inside surface transient temperature response is measured by a thin layer of choksteric liquid crystals. The color distribution oflhe liquid crysIIJIs is then translated Ihrough a digital color image processing unit into a surface temperature distribution. The local condensing heat transfer coefficients are obtained using a transient heal conduction analysis. Results for stratified flow regime are compared wilh results from the literature. This experimental technique provides a new tool for the measurement of lhe condensing local heal transfer.The technique is applicabk 10 measurements of condensing local heal transfer coefficients inside noncircular passages with plain or enhanced surfaces, which are commonly employed in state-of-me-an and advanced phase-change heal exchangers.

Determination of Incompressible Flow Friction in Smooth Circular and Noncircular Passages: A Generalized Approach Including Validation of the Nearly Century Old Hydraulic Diameter Concept

It has been demonstrated conclusively that the widely observed differences in data for frictional pressure coefficient between circular and noncircular passages derive from the inseparably connected effects of transition and the choice of a length scale. A relatively simple approach, the critical friction method (CFM), has been developed and when applied to triangular, rectangular, and concentric annular passages, the reduced data lie with remarkable consistency on the circular tube relations. In accordance with the theory of dynamical similarity, it has also been shown that noncircular duct data can be reduced using the hydraulic diameter or any arbitrarily defined length scale. The proposed method is what is needed to reconcile such data with those for circular tubes. With the hydraulic diameter, the critical friction factor almost converges to a universal value for all passages and the correction is simply that required to account for the difference in critical Reynolds number. By contrast, with any other linear parameter, two corrections are needed to compensate for variations in critical friction factor and Reynolds number. Application of the method to roughened passages is discussed.

Laminar Heat Transfer in Annular Sector Ducts

The continuing interest in compact heat exchangeers has created the need for friction factor and Nusselt number data for different passage shapes. It has long been recognized that circular tube results are generally not applicable to noncircular passages even when the hydraulic diameter is used as the characteristic dimension. Hence, design data should be generated for each passage individually, and a good source of such information is Shah and London. One duct geometry for which complete design information does not appear to be available in the open literature is that of annular sector ducts. Such configuration is encountered in multipassage internally finned tubes and many other compact het exchanger applications. The fluid flow problem for this configuration has been solved by Sparrow et al., and more recently by Niida. However, to the beest of the author's knowledge, the heat transfer results are not available yet. The purpose of this note is to summarize the analysis and results of fluid flow and heat transfer in annular sector ducts.

On the Equivalent Diameter Approach to Prediction of Rarefied Gas Flow in Noncircular Tubes

The feasibility of using a new characteristic dimension (equivalent diameter) is explored for predicting rarefied flows in noncircular tubes. A conclusion is that a single characteristic dimension is not necessarily appropriate. However, a normalized equivalent diameter does appear to give good relative descriptions of noncircular passages. In addition, a simplified equation based on the Weber equation is proposed. Coefficients in this equation include the variability of the characteristic dimension, and are determined by matching with three analytical solutions spanning the rarefied regimes. Significant computer time is saved and predictions are as accurate as the matching values. The classical hydraulic diameter appears to be the best characteristic dimension to use for correlating the predicted data.

Paper 12: Turbulent Flow and Heat Transfer in Concentric and Eccentric Annuli

Fluid flow and heat transfer in non-circular passages, with peripheral variation of heat flux around their boundaries, is of particular interest in the nuclear engineering field. This paper deals with heat transfer to air (Prandtl number ⋍ 0.7) in fully-developed turbulent flow in an annular passage, the outer boundary of which is insulated. Experiments have been conducted on annuii whose radius ratios are 1.632, 2.583, and 3.875 over a Reynolds number range 10 000–50 000. Pressure drop, velocity and temperature distributions in the cross-section have been measured over the whole range of eccentricity of the annulus geometry. The effect of eccentricity on average friction, average heat-transfer coefficient, and wall temperature distribution for uniform heat generation within the heated boundary has been studied. The results of the experimental programme are compared with the semi-theoretical predictions and experimental data of other workers in this field. For completeness, a theoretical equation for the heat-transfer coefficient in an internally heated concentric annulus has been derived.

Local Laminar Heat Transfer in Wedge-Shaped Passages

Abstract Heat transfer in noncircular passages recently has received increased attention in connection with various engineering applications, especially with nuclear-reactor technology. The present paper presents the results of calculations for heat transfer connected with steady laminar flow through ducts whose cross sections have the shape of circular sectors. Hydraulically and thermally developed flow is assumed. The temperature field, peripheral distribution of inner-duct surface temperature and of heat flux, as well as heat-transfer coefficients and friction factors, are presented for two boundary conditions: Peripherally constant heat flux and peripherally constant surface temperature, both with longitudinally constant heat flux. It is found that even average heat-transfer coefficients may be different by an order of magnitude for these two conditions. For this reason, it is pointed out that solutions for a class of boundary conditions can be obtained by linear superposition of the results obtained for the two conditions mentioned.

Flow in Corners of Passages With Noncircular Cross Sections

Abstract Conditions for convective heat transfer are known to be especially poor in the corner regions of noncircular passages. This situation causes the passage walls to become too hot in certain types of heat exchangers and directs attention to flow conditions in the corner regions. An experimental and theoretical investigation was conducted on the flow characteristics of triangular-shaped passages. Flow-visualization techniques revealed the remarkable fact that through a certain range of Reynolds numbers laminar and turbulent flows exist simultaneously side by side. Pressure measurements were made on two ducts to study the influence of the geometry on the hydrodynamic entrance length. The velocity field within the ducts was measured over a Reynolds number range from 500 to 43,000. An analytical study of the flow resulted in the development of a simple expression to describe the velocity field under the conditions of laminar fully developed flow in a narrow corner. This expression was confirmed experimentally using a sensitive micromanometer developed for this purpose.