Local Heat Transfer Coefficient Distribution Research Articles

Local heat-transfer coefficient estimation in cross-helix corrugated tubes under turbulent regime

In a wide variety of engineering applications, convection heat transfer enhancement plays a major role in reducing the heat exchanger’s size and costs and increasing their efficiency. The passive methods for increasing convective heat transfer are the most interesting since they don’t require any external power to achieve the desired enhancement. For this reason, this approach is preferred in the industrial sector of heat exchangers. For the enhancement of the performances of these apparatuses, in particular, the tubular ones used in the pharmaceutical, food, and chemical, the most adopted passive solution is the corrugation of their tube walls. Two pipes characterized by cross-helix corrugation have been experimentally analyzed both investigating the global heat transfer performance and studying the local heat transfer coefficient distribution to profoundly examine the thermal performance enhancement mechanisms of this passive technique. It has been observed that at Re = 16,000, the heat transfer rate in the smaller pitch tube is 18.18% higher than that in the larger pitch tube. The inverse heat conduction problem in the tube wall is addressed to achieve the local coefficient distribution method by using temperature readings collected from an infrared camera on the outer surface of the tube as the initial data.

Cooling Characteristics of the Hot-Rolled Seamless Steel Tube Impinged via Inclined Jet

The characteristics of flow field distribution and temperature variation of an inclined jet impinging on a steel tube surface at different positions in circumferential directions were studied via numerical simulation. By analyzing the local convective heat transfer coefficient in circumferential direction, it was shown that the downstream and upstream regions had the characteristics of typical asymmetry. As the inclination angle increases, the local convective heat transfer coefficient gradually increases in the downstream region and gradually decreases in the upstream region. When the θ of the top and bottom jet is 30°, the increases in the downstream region are 40.2% and 54.6%, respectively. Based on the study of the local convective heat transfer coefficient and temperature distribution in the circumfluence direction of a steel tube during the cooling process, it was shown that the optimal inclination angle is 0~10°. With the increase in inclination angle, the average heat transfer coefficient shows a decreasing trend overall. With the increase in jet Reynolds number, the decrease in the average heat transfer coefficient gradually decreases. When the inclination angle increases to 30°, the effect of inclination angle on steel tube cooling is obviously stronger than that of jet position.

Experimental assessment of the gap width effect on turbulent flow and forced convective heat transfer around a single rod suspended in a channel

The present work presents the experimental campaign carried out to assess the convective heat transfer around a single rod suspended in a rectangular channel under axial turbulent flow. The distance between the rod and the upper wall of the channel forms a rod-wall gap, whose width, d, can be varied. The experiments were performed for the dimensionless gap widths that yield W/D = 1.050, 1.100, 1.150 and 1.200. Hot-wire anemometry was used to measure mean velocities, the RMS velocity fluctuations and the velocity spectra. The rod is equipped with a heated cell that provides the local temperature and convective heat transfer coefficient distributions around the rod’s surface. The local convective heat transfer distribution is in good agreement with the literature. The lowest value for the heat transfer coefficient was not found right in the narrow gap for W/D = 1.050, but, slightly away from the gap. As W/D increases, the minimum local convective heat transfer coefficient occurred closer to the narrowest gap width. The Reynolds-Colburn analogy was investigated for all configurations through the measured convective heat transfer coefficient and the skin friction distributions. A linear relationship was found between the Stanton number and the skin friction coefficient. Moreover, data scattering was seen to decrease significantly when the local velocity was used instead of the global one. It suggests that the heat transfer process is governed, mainly, by the local parameters.

Numerical simulation of falling film sensible heat transfer over round horizontal tubes

Numerical simulations are performed to explore the heat transfer characteristics of falling films over horizontal round tubes with uniform heat flux imposed for a range of Reynolds numbers spanning the droplet, jet, and sheet regimes. Simulations results agree well with available experimental measurements. The study analyzes the local as well as average heat transfer behavior under the different flow modes. The numerical results show that the local Nusselt number (Nu) distribution depends on the flow features in each mode and varies substantially in all directions for the respective mode. In the droplet mode, the Nu value varies significantly as the droplet impinges and the remnant liquid-bridge retracts (peak instantaneous Nu near 6), followed by wave propagation over the tube surface with peak Nu around 0.25. For the jet modes, the local maximum in the heat transfer occurs off-center to the impingement location with magnitudes of peak Nu = 3.1 for the inline jet mode and Nu = 2.7 for the staggered jet mode, while on the rest of the tube surface, it has an inverse relation with the liquid film thickness. Substantial variations in the heat transfer value are also recorded in the middle of the two impinging jets with Nu = 0.95 in the inline jet mode where the neighboring jets do not interact, and Nu = 0.60 in the crest region of the staggered jet mode where the neighboring jets interact with each other. In the sheet mode, the Nu was seen to depend on the thickness of the liquid waves traversing over the tube surface. Lower Nu values were recorded beneath the crest location of the liquid waves, which increases (1.4–11.6% depending on circumferential location) abruptly in magnitude at the advancing fronts of the waves. The temperature distribution in the liquid film in each of the modes was examined to evaluate the mechanism of heat transfer process. This study also compares the local heat transfer coefficient distribution with the analytical heat transfer models derived to predict heat transfer performance over horizontal tube surfaces.

Effect of winding angles on flow and heat transfer characteristics in the shell side of spiral wound heat exchangers

A three-dimensional model is established to investigate the flow and heat transfer characteristics influenced by the winding angle in the shell side of a spiral wound heat exchanger. Numerical simulations are conducted with the winding angle varying from 0° to 20° and the Reynolds number varying from 430 to 1935. The flow states of different conditions are observed, and the film thickness is measured in the axial and circumferential directions. Moreover, the heat transfer characteristics of the liquid falling film flow are investigated at constant wall temperature. The results indicate that the winding angle has a distinct effect on the flow and heat transfer characteristics. Under the same Reynolds number, the local film thickness distribution varies with the winding angle, and the average film thickness in the axial direction decreases with the increase of winding angle. The local heat transfer coefficient distribution is consistent with that of film thickness, and the average heat transfer coefficient increases with the Reynolds number but decreases with the winding angle. Based on the results, a comprehensive heat transfer correlation involving winding angles is developed, the deviations of the data are within ± 3%.

Validation of RANS Turbulence Models for the Conjugate Heat Exchange Problem

This paper addresses problems of mathematical modeling of heat exchange processes in the pre-nozzle volume of a solid propellant rocket engine with a charge with starlike cross-section and a recessed hinged nozzle. Methods of mathematical modeling are used to solve the quasi-stationary spatial conjugate problem of heat exchange. An analysis is made of the influence of RANS turbulence models on the flow structure in the flow channels of the engine and on the computed heat flow distributions over the surface of the recessed nozzle. Methods of mathematical modeling are used to solve the quasi-stationary spatial conjugate problem of heat exchange. Results of validation of RANS turbulence models are presented using well-known experimental data. A comparison of numerical and experimental distributions of the heat-transfer coefficient over the inlet surface of the recessed nozzle for the engine with a cylindrical channel charge is made for a primary choice of turbulence models providing a qualitative agreement between calculated and experimental data. By analyzing the results of numerical modeling of the conjugate problem of heat exchange in the combustion chamber of the solid propellant engine with a starlike channel, it is shown that the SST $k-\omega$ turbulence model provides local heat-transfer coefficient distributions that are particularly close to the experimental data.

Endwall heat transfer characteristics of octahedron family lattice-frame materials

Recent studies on traditional metal foam thick blocks suggest that much of the foam volume remains unused in heat dissipation and that the thinner configurations are more efficient in forced convection scenarios. Single cell thick lattice-frame configurations are attractive solutions to the problem of unused foam volume and can also be additively manufactured through Direct Metal Laser Sintering process while also offering superior load bearing capabilities. Due to the inherent nature of such single cell thick configurations, their unit cell topologies' footprint at the enclosing walls has significant effect in both local heat transfer coefficient distribution and net overall heat dissipation. This study presents detailed endwall heat transfer coefficient measurements for lattice-frame structure of Octet unit cell topology and compares its thermal performance with two other members of the Octahedron family, viz. Octa and V-Octet. Developing heat transfer was measured through transient liquid crystal thermography at the endwalls of a unity aspect ratio duct featuring five consecutive interconnected unit cells at a Reynolds number of 37,200. Relative to a smooth channel, the cell-based averaged Nusselt numbers for Octet, Octa and V-Octet were 2.96, 2.78 and 3.05 times, respectively.

Unsteady Analysis of Heat Transfer Coefficient Distribution in a Static Ribbed Channel for An Established Flow

Abstract Turbulent-ribbed channels are extensively used in turbomachinery to enhance convective heat transfer in internally cooled components such as turbine blades. One of the key aspects of such a problem is the distribution of the heat transfer coefficient (HTC) in fully developed flows, and many studies have addressed this problem by the use of computational fluid dynamics (CFD). In the present document, large eddy simulation (LES) is performed for a configuration from a test-rig at the Von Karman Institute representing a square channel with periodic square ribs. The whole channel is computed in an attempt to better understand HTC maps in this specific configuration. Resulting mean and unsteady flow features are captured, and predictions are used to further explain the obtained HTC distribution. More specifically turbulent structures are seen to bring cold gas from the main flow to the wall. A statistical analysis of these events using the joint velocity-temperature probability density function (PDF), and quadrant method allows to define four types of events happening at every location of the channel and which can then be linked to the HTC distribution. First, the HTC is very high where the flow impacts the wall with cold temperature whereas it is lower where the hot gas is ejected to the main flow. In an attempt to link the HTC trace on the channel wall with structures in the flow field far-off the wall, the main modes are identified performing power spectral density (PSD) analysis of the velocity along the channel. Dynamic mode decomposition (DMD) of the flow field data is then used to present the spatio-temporal characteristics of two of the identified most dominant modes: a vortex-street mode linked to the first rib and a rib-to-rib mode appearing because of the quasi-periodicity of the configuration. However, DMD analysis of the HTC trace on the wall does not emphasize any dominant mode. This indicates a weak link between the main flow large scale features and the instantaneous and more local HTC distribution.

Experimental study on the distribution of local heat transfer coefficient of falling film heat transfer outside horizontal tube

An experimental system was built up to measure the outer circumferential distributions of the local heat transfer coefficient of horizontal tube falling film flow. The wall temperature along the angle were measured. Based on the experimental data, the local heat flux distribution on the tube surface were obtained by numerical calculation. The local heat transfer coefficient distribution outside the tube were obtained. The effects of spray density, tube spacing, average heat flux and fluid temperature on local heat transfer coefficient were investigated. The results show that the local heat transfer coefficient decreases sharply with the increase of angle, then decreases steadily, and finally increases slightly. According to the distribution of the local heat transfer coefficient and heat transfer mechanisms, the tube surface is divided into the impingement zone, the heat diffusion transfer zone and the tail detachment zone. The increase of spray density has little influence on the heat transfer in the impact zone, but make the heat transfer increases slightly in the heat diffusion transfer zone and the tail detachment zone. The influence of tube spacing is mainly concentrated in the impingement zone. The heat transfer is enhanced with the increase of tube spacing in the impingement zone. The average heat flux has no obvious effect on heat transfer. With the increase of fluid temperature, the heat transfer is greatly increased all over the cycle. Besides, based on the experimental data, new correlations of local heat transfer coefficient of falling film flow outside a horizontal tube are proposed.

Düz Bir Plaka Üzerindeki Hidrodinamik ve Isıl Sınır Tabaka Akışının Sayısal Olarak İncelenmesi ve Geçiş Kontrolü

In this study, Computational Fluid Dynamics (CFD) calculations are performed with ANSYS-Fluent for an external flow over a flat plate under constant surface temperature conditions. By using an Active Flow Control (AFC) method, the flat-plate is heated to manipulate the transition region. Calculations are performed for a steady and turbulent flow at 15 m/s free-stream velocity. Local skin friction coefficient and local heat transfer coefficient distributions along the flat-plate are investigated for laminar and turbulent boundary layers at various constant surface temperatures. For laminar and turbulent flow boundary layer characteristics, theoretical correlations in the literature are used to verify the numerical results. Results show that theoretical correlations are highly consistent with CFD results only in the laminar and turbulent regions and it is also shown that transition can only be predicted by CFD simulations. On the other hand, heating as an AFC method is found to be useful in delaying transition regime over a flat plate.

Dynamic behaviour of a plate heat exchanger: Influence of temperature disturbances and flow configurations

Understanding the dynamic properties of a plate heat exchanger plays an essential role when developing smart control algorithms for modern energy-efficient district heating and cooling applications. In this work, the dynamic behaviour of a counterflow plate heat exchanger is studied in order to understand its transient responses due to inlet temperature disturbances at different fluid flow configurations. For this purpose, a suitable theoretical model has been proposed. The temperature transients are evaluated numerically by solving a lumped-parameter system representing the derived 1D model of the studied plate heat exchanger. The numerical results revealed that different fluid flow configurations considerably influence the transient temperature responses as well as the overall temperature drop and heat transfer rate when following the cold down process of a fluid travelling from the inlet to the outlet of the plate heat exchanger hot fluid side. The predicted transients are experimentally verified by conducting a series of systematic tests using infrared thermography technology. The analysis shows that the proposed model is in good agreement with the experiments. The results of the thermal imaging measurements also provide a more in-depth insight into the temperature distribution and its transient front propagation along the fluid flow channels. In future studies, such an experimental approach is valuable to identify the critical temperature zones when predicting the thermal fatigue life-time of brazed plate heat exchangers and as well to extend the proposed theoretical model to include the 2D local heat transfer coefficient distribution obtained by the IR thermography.

Local Heat Transfer Distributions Within a Rotating Pin-Finned Brake Disk

Abstract This study presents local temperature and heat transfer coefficient distributions obtained experimentally on the internal surfaces of a rotating pin-finned brake rotor at realistic rotation speeds for braking (i.e., N = 100–300 rpm). To this end, the thermochromic liquid crystal technique in a rotating reference frame was employed. The results demonstrate that the bulk airflow within the ventilated channel of a rotating disk follows a predominantly backward sweeping inline-like path between the pin fins. Internal local heat transfer is distributed nonuniformly on both inboard and outboard surfaces, with twice higher average cooling from the outboard surface than the inboard surface: this possibly exacerbates the thermal stresses, which leads to thermal distortion of the rotor (i.e., coning).

Heat Transfer Characteristics of Microjet Impingements with Flow Extraction

The present study proposes a novel microjet impingement-effusion model for electronics cooling. The model is realized on a silicon substrate with the arrangement of impinging and extraction holes. A conjugate heat transfer analysis is carried out which includes heat transfer from the substrate base to the impinging fluid which subsequently taken away through the extraction holes. The impingement microjet is surrounded by six micro holes distributed hexagonally to extract the spent flow. A three-dimensional (3-D) numerical analysis is performed for a single-phase steady state laminar flow in view of the small flow rates and micro scales. The proposed model is investigated for local temperature and heat transfer coefficient distribution. The highest temperature is observed near the impingement, and the lowest temperature is observed on the surface below the extraction holes. With the increase of jet-to-effusion hole diameter square ratio, the high-temperature region is reduced. However, there is no significant improvement is detected in the local heat transfer coefficient.

Heat transfer characteristics of an ex-vessel molten core cooling system based on two-phase natural circulation

Passive cooling based on natural circulation is utilized in ex-vessel core catcher system of an advanced nuclear reactor to handle severe accident scenario. The core catcher coolant channel has a unique geometry which consists of heated downward-facing slightly inclined and vertical surfaces. A full height experimental facility with natural circulation driven flow to model ex-vessel core catcher system was designed using scaling analysis. In this study, the cooling capability and heat transfer characteristics of the ex-vessel core catcher system was carried out. Two-phase flow parameters and wall temperatures were measured under a uniform heat flux condition. Two-phase flow structures were identified by high-speed camera visualization along with measurements of two local parameters, void fraction and re-wetting time. The wall temperature and local heat transfer coefficient distribution along the cooling channel were obtained by direct measurements of the heater surface and liquid temperature. The cooling performance of the core catcher system was analyzed based on the experimental results. The results indicated that the core catcher coolant system provides adequate cooling and maintains the integrity of the core catcher plate for prototype heat flux conditions.

Experimental study of the transitional flow regime in coiled tubes by the estimation of local convective heat transfer coefficient

The present work is focused on studying the transition between the laminar and turbulent flow regime in coiled tubes. Wall curvature is a popular heat transfer enhancement technique since it gives origin to a secondary flow in the flowing fluid due to the non-uniformity of the centrifugal force over the cross section. This phenomenon, both in the laminar and the turbulent flow regime, promotes local maxima in the velocity distribution that locally increase the temperature gradient at the wall by enhancing the heat transfer and, at the same time, leading to a significant variation in the convective heat-transfer coefficient along the circumferential angular coordinate. However, this geometry delays the transition from laminar to turbulent regime, transition that, in the majority of the papers available in the scientific literature, has been investigated on the basis of pressure drop data behaviour. In the present work the estimation of the local convective heat transfer coefficient distribution, based on the solution of the inverse heat conduction problem in the tube’s wall, is proposed as a complementary and detailed tool to investigate the transitional flow regime. Moreover, the present research, thanks to the application of the proposed approach to an experimental case, gives additional information on the phenomenon of transition in coiled tubes.

Heat Transfer Performance of Internal Cooling Channel With Single-Row Jet Impingement Array by Varying Flow Rates

The current detailed experimental study focuses on the optimization of heat transfer performance through jet impingement by varying the coolant flow rate to each individual jet. The test section consists of an array of jets, each jet individually fed and metered separately, that expel coolant into the channel and exit through one end. The diameter D, height-to-diameter H/D, and jet spacing-to-diameter S/D are all held constant at 9.53 mm, 2, and 4, respectively. Upon defining the optimum flow rate for each jet, varying diameter jet plates are designed and tested using a similar test setup with the addition of a plenum. Two test cases are conducted by varying the jet diameter within 10% compared to the benchmark jet diameter, 9.53 mm. The Reynolds number, which is based on hydraulic diameter of the channel and total mass flow rate entering the channel, ranges from approximately 52,000 up to 78,000. The transient liquid crystal technique is employed in this study to determine the local and average heat transfer coefficient distributions on the target plate. Commercially available computational fluid dynamics software, ansys cfx, is used to qualitatively correlate the experimental results and to fully understand the flow field distributions within the channel. The results revealed that varying the jet flow rates, total flow varied by approximately ±5% from that of the baseline case, the heat transfer enhancement on the target surface is enhanced up to approximately 35%. However, when transitioning to the varying diameter jet plate, this significant enhancement is suppressed due to the nature of flow distribution from the plenum, combined with the complicated crossflow effects.

Effects of surface roughness on the heat transfer characteristics of water flow through a single granite fracture

The understanding of flow and heat transfer in a rough fracture is not sufficient at present due to the complexity of quantifying fracture geometry and the lack of experimental work. This paper adopted fractal dimension D and profile waviness Ra to characterize surface roughness and investigated the effects of surface roughness on the heat transfer characteristics of water flow through a single granite fracture by combining the experimental and numerical modeling approaches. It is found that the local heat transfer coefficient distribution mainly depends on the fracture surface roughness, followed by aperture and flow rate.

Prediction of convective heat transfer coefficient of human upper and lower airway surfaces in steady and unsteady breathing conditions

Bio-effluent and metabolic heat production from the human body and its breathing activity can strongly influence the microclimate around the body. On the other hand, local properties of the microclimate around the human body can also significantly affect the interaction between the body and the surrounding environment by way of local flow and heat transfer characteristics close to the body. Breathing is one of the most essential activities in our lives, and the basic functions of breathing include exchanging gases (supplying oxygen from ambient air and removing carbon dioxide from the blood) and exchanging heat and moisture (sensible and latent heat). As a consequence, human beings experience lifelong interaction with indoor environments via inhalation. In this study, two types of three-dimensional respiratory tract models were developed using computed tomography data of a healthy human males. Computational fluid dynamics simulations are performed to analyze the airflow and temperature distributions inside respiratory tract models under various breathing conditions. We used low-Reynolds-number-type k-ε model to predict airflow in the airway models. The flow pattern inside the viscous sub-layer and convective heat flux on the airway tissue surfaces and convective heat transfer coefficients were analyzed. Through this study, the numerical errors were successfully identical, so this discrepancy of two airway models were assumed due to the differences in airway geometries and reflected individual specificity. Averaged and local convective heat transfer coefficient distributions of the human airway were summarized as functions of breathing airflow rate.

The Relative Performance of External Casing Impingement Cooling Arrangements for Thermal Control of Blade Tip Clearance

As a key way of improving jet engine performance, a thermal tip clearance control system provides a robust means of manipulating the closure between the casing and the rotating blade tips, reducing undesirable tip leakage flows. This may be achieved using an impingement cooling scheme on the external casing. Such systems can be optimized to increase the contraction capability for a given casing cooling flow. Typically, this is achieved by changing the cooled area and local casing features, such as the external flanges or the external cooling geometry. This paper reports the effectiveness of a range of impingement cooling arrangements in typical engine casing closure system. The effects of jet-to-jet pitch, number of jets, and inline and staggered alignment of jets on an engine representative casing geometry are assessed through comparison of the convective heat transfer coefficient distributions as well as the thermal closure at the point of the casing liner attachment. The investigation is primarily numerical, however, a baseline case has been validated experimentally in tests using a transient liquid crystal technique. Steady numerical simulations using the realizable k–ε, k–ω SST, and EARSM turbulence models were conducted to understand the variation in the predicted local heat transfer coefficient distribution. A constant mass flow rate was used as a constraint at each engine condition, approximately corresponding to a constant feed pressure when the manifold exit area is constant. Sets of local heat transfer coefficient data generated using a consistent modeling approach were then used to create reduced order distributions of the local cooling. These were used in a thermomechanical model to predict the casing closure at engine representative operating conditions.

Schlieren visualization of water natural convection in a vertical ribbed channel

Schlieren techniques are valuable tools for the qualitative and quantitative visualizations of flows in a wide range of scientific and engineering disciplines. A large number of schlieren systems have been developed and documented in the literature; majority of applications involve flows of gases, typically air. In this work, a schlieren technique is applied to visualize the buoyancy-induced flow inside vertical ribbed channels using water as convective fluid. The test section consists of a vertical plate made of two thin sheets of chrome-plated copper with a foil heater sandwiched between them; the external sides of the plate are roughened with transverse, square-cross-sectioned ribs. Two parallel vertical walls, smooth and unheated, form with the heated ribbed plate two adjacent, identical and asymmetrically heated, vertical channels. Results include flow schlieren visualizations with colour-band filters, reconstructions of the local heat transfer coefficient distributions along the ribbed surfaces and comparisons with past experiments performed using air as working fluid.