Showing papers in "Heat Transfer Engineering in 2013"

Review of Molten-Salt Thermocline Tank Modeling for Solar Thermal Energy Storage

Abstract: Molten-salt thermocline tanks are a low-cost option for thermal energy storage in concentrating solar power systems. A review of previous experimental and numerical thermocline tank studies is performed to identify key issues associated with tank design and performance. Published models have shown that tank discharge performance improves with both larger tank height and smaller internal filler diameter due to increased thermal stratification and sustained outflow of molten salt with high thermal quality. For well-insulated (adiabatic) tanks, low molten-salt flow rates reduce the axial extent of the heat-exchange region and increase discharge efficiency. Under nonadiabatic conditions, low flow rates become detrimental to stratification due to the development of fluid recirculation zones inside the tank. For such tanks, higher flow rates reduce molten-salt residence time inside the tank and improve discharge efficiency. Despite the economic advantages of a thermocline tank, thermal ratcheting of the tank wa...

A Critical Review of Basic Crystallography to Salt Crystallization Fouling in Heat Exchangers

Abstract: Fouling formation on heat exchanger surfaces due to crystallization of inverse solubility salts is one of the fundamental problems in process industries. Despite numerous studies carried out in recent years, comprehensive understanding of crystallization fouling mechanism remains a challenge to chemical engineers. In this review, we first focus on the basic crystallography during deposition of calcium salts, paying attention to crystal structures and crystal forms, as well as nucleation and the subsequent crystal growth process. We then endeavor to relate a number of factors to fouling rate, which may be classified into three categories: solution composition, operating parameters, and heat exchanger surface characteristics. Each aspect is discussed from the crystallization viewpoint (science) and in terms of possible industrial applications (practice). Combining the basic knowledge of crystallography with the information from experimental investigations, several fouling mitigation methods have also been described that may reduce fouling. It is hoped that some of the ideas discussed here will provide possible economic and environmental benefits. Finally, we also try to throw some light on the future direction for research.

Advances in Electronics Cooling

Abstract: This article highlights the advantages of on-chip microchannel cooling technology, based on first- and second-law analysis and experimental tests on two types of cooling cycles, the first driven by an oil-free liquid pump and the second by an oil-free vapor compressor. The analysis showed that the drivers of the fluid were the main culprits for major losses. It was further found that when energy recovery is of importance, making use of a vapor compression cycle increases the quality of the recovered energy, hence increasing its value. This was demonstrated by analyzing the synergy that can exist between the waste heat of a data center and heat reuse by a coal-fired power plant. It was found that power-plant efficiencies can be increased by up to 6.5% by making use of a vapor compression cycle, which results not only in significant monetary savings, but also in the reduced overall carbon footprints of both the data center and the power plant.

Constructal Optimization of Microchannel Heat Sinks With Noncircular Cross Sections

Abstract: This article reports the numerical geometric optimization of three-dimensional microchannel heat sinks with rectangular, elliptic, and isosceles triangular cross sections. The cross-sectional areas of the mentioned microchannels can change according to the degrees of freedom, that is, the aspect ratio and the solid volume fraction. Actually, the purpose of geometric optimization is to determine the optimal values of these parameters in such a way that the peak temperature of the wall is minimized. The effects of solid volume fraction and pressure drop upon the aspect ratio, hydraulic diameter, and peak temperature of the microchannels are investigated. Moreover, these microchannel heat sinks are compared with each other at their optimal conditions. Considering the constraints and geometric parameters for the optimization of the present study, it is revealed that microchannel heat sinks with rectangular and elliptic cross sections have similar performances, while microchannels with isosceles triangular cro...

Fundamental Issues of Critical Heat Flux Phenomena During Flow Boiling in Microscale-Channels and Nucleate Pool Boiling in Confined Spaces

Abstract: This article presents a comprehensive literature review on the fundamental issues of critical heat flux (CHF) during flow boiling and nucleate pool boiling in microscale channels and confined spaces. First, distinction between macro- and micro-scale channels is discussed. Then the CHF mechanisms are discussed. Next, experimental and theoretical studies of subcooled flow boiling CHF in microscale channels together with the prediction methods are reviewed and analyzed. Following this, experimental and theoretical studies on saturated flow boiling CHF together with the prediction methods are summarized and discussed. Furthermore, experimental and theoretical studies on nucleate pool boiling CHF in confined spaces together with the relevant prediction methods are reviewed as well. So far, limited studies on CHF microscale channels and confined spaces are available in the literature. There are numerous discrepancies in the existing studies on CHF results, mechanisms, and prediction methods. Furthermore, there ...

Polymer nanowire arrays with high thermal conductivity and superhydrophobicity fabricated by a nano-molding technique

Abstract: High thermal conductivity is helpful for thermal control and management, and superhydrophobicity can benefit fluid friction reduction and liquid droplet control in micro-/nanodevices. We report on a nano-molding technique that can prepare polyethylene nanowire arrays with high thermal conductivity (more than 10 W/m-K) and superhydrophobicity (contact angle >150°). The thermal conductivities of the fabricated high-density polyethylene nanowire arrays with diameters of 100 nm and 200 nm, measured by a laser flash method, are about 2 orders of magnitude higher than their bulk counterparts. The estimated thermal conductivity of a single high-density polyethylene nanowire is as high as 26.5 W/m-K at room temperature, while the thermal conductivity of low-density polyethylene nanowire is a little smaller. The self-organized surfaces of polymer nanowire arrays are found to have micro-to-nanoscale hierarchical nanostructures, and have superhydrophobicity of greater than 150° contact angles for water. We also meas...

Single-Phase Convective Heat Transfer and Pressure Drop Coefficients in Concentric Annuli

Abstract: Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however, these are complex and time-consuming, and require much experimental data in order to obtain accurate solutions. A large data set of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593, and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm, and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a nonlinear regression scheme, and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using ...

Development of a Predictive CFD Fouling Model for Diesel Engine Exhaust Gas Systems

Abstract: This article presents a numerical simulation procedure for studying soot particle deposition in diesel exhaust systems, with a particular focus on fouling layer thickness evolution. In the proposed algorithm, particle transport toward the wall, adhesion, and reentrainment of particles from the surface have been modeled, including Brownian motion and turbulent diffusion, thermophoresis, adhesion, and removal. This model has been implemented in ANSYS Fluent, which makes the inclusion of local effects possible. A cross-flow device, with a tube positioned transverse to the flow, has been simulated and tested. A comparison of the predicted fouling layer at several angular positions with the experimental observation shows acceptable agreement. This model makes it possible to predict the real depth of the fouling layer and its effects on the hydrodynamics of the flow. This model represents a valuable tool for the prediction of the main aspects of the performance of heat exchangers exposed to fouling.

Optimization of Microchannel Heat Sinks Using Genetic Algorithm

Abstract: In this study, a genetic algorithm is employed to minimize the entropy generation rate in microchannel heat sinks. The entropy generation rate allows the combined effects of thermal performance and pressure drop to be assessed simultaneously as the heat sink interacts with the surrounding flow field. Previously developed models for the heat transfer, pressure drop and entropy generation rate are used in the optimization procedure. The results of optimization are compared with existing results obtained by the Newton–Raphson method. It is observed that the GA gives better overall performance of the microchannel heat sinks.

Experimental Investigation of Inertial Mixing in Colliding Droplets

Abstract: Achieving the increasingly fast mixing requirements posed by the chemical, biological, and life science community for confined microchannel droplet flows remains an engineering challenge. The viscous and surface tension forces that often dominate microflows undermine fast, efficient mixing. A novel mixing arrangement based on droplet collisions has been developed that significantly improves mixing rates by utilizing inertia to rapidly rearrange fluid contents. This article experimentally investigates inertial droplet mixing in micro-flows following high-speed droplet pair collisions. The technique utilizes a gaseous flow for liquid droplet generation and transport with collisions occurring in Y-junction microchannel geometries. Mixing rates are quantified using differential fluorescent optical diagnostics, custom image processing algorithms, and statistical analysis. Measured droplet mixing times are compared to the characteristic time scales for mass and viscous diffusion and bulk convective transport. R...

Heat Transfer in Water-Based SiC and TiO2 Nanofluids

Abstract: The article reports the results of heat transfer experimental tests on water-based TiO2 (9 wt%) and SiC (3, 6, 9 wt%) nanofluids. Measurements were performed in a two-loop test rig for immediate comparison of the thermal performances of the nanofluid with the base fluid. The convective heat transfer is evaluated in a circular pipe heated with uniform heat flux (from 20 to 240 kW/m2) and flow regimes from laminar to turbulent. Tests have been performed to compare the heat transfer of nanofluids and water at the same velocity (from 0.7 to 1.6 m/s) or Reynolds number (from 300 to 6000), and they have also been compared with values calculated from some of the most widely used correlations. The analysis of the experimental data shows a strong dependence on the parameter used, while both the nanofluid and water data have the same agreement with the calculated values. Nanofluids were manufactured through a two-step procedure: laser synthesis of nanoparticles followed by dispersion in water.

Performance Comparison of Three-Bed Adsorption Cooling System With Optimal Cycle Time Setting

Abstract: This article presents the optimal cycle time and performance of two different types of silica gel–water-based three-bed adsorption chillers employing mass recovery with heating/cooling scheme. A new simulation program has been developed to analyze the effect of cycle time precisely on the performance of the systems. The particle swarm optimization (PSO) method has been used to optimize the cycle time and then the optimum performances of two chillers are compared. Sensitive analysis of cycle time has been conducted using the contour plot of specific cooling power (SCP) with driving heat source temperature at 80°C. It is found that the center point of the contour indicates the maximum SCP value and optimal cycle time, which are comparable with the quantitative values obtained for the PSO method. Both three-bed mass recovery adsorption cycles can produce effective cooling at heat source temperature as low as 50°C along with a coolant at 30°C. The optimal SCP is similar for both cycles and is greater than tha...

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Abstract: Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m2-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m2. The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer det...

The Effect of Alumina–Water Nanofluid on Natural Convection Heat Transfer Inside Vertical Circular Enclosures Heated from Above

Abstract: Experimental investigation on natural convection heat transfer is carried out inside vertical circular enclosures filled with Al2O3–water nanofluid with different concentrations; 0.0%, 0.85% (0.21%), 1.98 (0.51%), and 2.95% (0.75%) by mass (volume). Two enclosures are used with 0.20 m inside diameter and with two different aspect ratios. The top surface of the enclosure is heated using a constant-heat-flux flexible foil heater while the bottom surface is subject to cooling using an ambient air stream. Various heat fluxes are used to generate heat transfer through the nanofluid. The average Nusselt number is obtained for each enclosure and correlated with the modified Rayleigh number using the concentration ratio as a parameter. A general correlation for the average Nusselt number with the modified Rayleigh number is obtained using the volume fraction and the aspect ratio as parameters to cover both enclosures. The results show that the Nusselt number for the alumina–water nanofluid is less than that of th...

Effect of Twin Delta-Winged Twisted-Tape on Thermal Performance of Heat Exchanger Tube

Abstract: This work aims at studying the effect of twin delta-winged twisted-tape insertion on heat transfer, pressure drop, and thermal performance characteristics of a heat exchanger tube. All twisted tapes used in this work were made of aluminum sheets twisted at a single twist ratio of 3.0. The twin delta wings were formed by extrusion of the tape at the center area at every twist length interval. For comparison, three different arrangements of the twin delta wings were: (1) the wing tips pointing upstream of the flow (TTW-up, twin delta-winged twisted tape in counterflow arrangement), (2) the wing tips pointing downstream of the flow (TTW-down, twin delta-winged twisted tape in co-flow arrangement), and (3) the wing tips pointing opposite direction (TTW-o, opposite winged twisted tape). The wing declination was arranged at an angle of 15° with respect to the tape surface. Effects of three different wing-tip angles of 20°, 40°, and 60° for a constant wing base were examined. The experiments were conducted using...

Effect of Void Fraction and Two-Phase Dynamic Viscosity Models on Prediction of Hydrostatic and Frictional Pressure Drop in Vertical Upward Gas–Liquid Two-Phase Flow

Abstract: In gas–liquid two-phase flow, the prediction of two-phase density and hence the hydrostatic pressure drop relies on the void fraction and is sensitive to the error in prediction of void fraction. The objectives of this study are to analyze dependence of two-phase density on void fraction and to examine slip ratio and drift flux model-based correlations for their performance in prediction of void fraction and two-phase densities for the two extremes of two-phase flow conditions, that is, bubbly and annular flow or, alternatively, the low and high region of the void fraction. It is shown that the drift flux model-based correlations perform better than the slip ratio model-based correlations in prediction of void fraction and hence the two-phase mixture density. Another objective of this study is to verify performance of different two-phase dynamic viscosity models in prediction of two-phase frictional pressure drop. Fourteen two-phase dynamic viscosity models are assessed for their performance against 616 d...

Parameter Estimation of Fouling Models in Crude Preheat Trains

Abstract: Several fouling mitigation techniques depend on the capacity of predicting fouling rates. Therefore, the identification of accurate fouling rate models is an important task. Crude fouling rates are usually evaluated through empirical or semiempirical models. In both alternatives, there are parameters that must be determined through laboratory or process data. In this context, the article presents an analysis of the parameter estimation problem involving fouling rate models. A proposed procedure for addressing this problem is described through the development of a computational routine called HEATMODEL. An important aspect of this study is focused on the obstacles associated to the search for the optimal set of parameters of the Ebert and Panchal models and its variants. This optimization problem may present some particularities that complicate the utilization of traditional algorithms. In the article, the performance of a conventional optimization algorithm (Simplex) is compared with a more modern numeric...

Condensation, Frost Formation, and Frost Melt-Water Retention Characteristics on Microgrooved Brass Surfaces Under Natural Convection

Abstract: In this study, frost is grown on microgrooved and flat brass samples under specific operating conditions and a comparison of the condensation, frosting, and defrosting pattern on microgrooved and flat brass surfaces is carried out experimentally. The surfaces are fabricated by a mechanical micromachining process and no chemical alteration of the surface is conducted. It is found that the shape, size, and distribution of condensed water droplets and the subsequent frost structure are significantly affected by the presence of microgrooves on the surface. The condensed water droplets take an elongated shape and then coalesce along the pillars and grooves on the microgrooved surfaces, giving a parallel “brick-wall-like” frosting pattern. The frost crystals on these microgrooved surfaces exhibit more directional growth parallel to the surface, with numerous ice flakes growing in the perpendicular and angular directions to the grooves. This nonuniform growth of the frost layer also gives the appearance of a spo...

Nanoparticle-Related Heat Transfer Phenomenon and Its Application in Biomedical Fields

Abstract: Recent years have seen intensive interest in the synthesis and application of nanomaterials in different fields extending from energy to biomedicine sectors. Many biomedical applications involve delivering biomodified nanoparticles to malignant cells and rapidly heating nanoparticles with an external source such as laser, ultrasound, or an electromagnetic wave to produce a therapeutic thermal effect or to release drugs. The interaction of nanoparticles with the external source and the subsequent heating effect are fundamental for the successful deployment of these novel techniques. This study reviews nanoparticle-related heat transfer phenomenon and its medical applications. Some initial theoretical and experimental studies are conducted to reveal the potentials of this exciting field. The combination of gold nanoparticles with ultrasound irradiation or electromagnetic waves in the radiofrequency spectrum has been shown to be a promising strategy for future medical applications. Further understanding of t...

Design of Flat-Plate Dehumidifiers for Humidification–Dehumidification Desalination Systems

Abstract: Flat-plate heat exchangers are examined for use as dehumidifiers in humidification–dehumidification (HDH) desalination systems. The temperature and humidity ratio differences that drive mass transfer are considerably higher than in air-conditioning systems, making current air-conditioning dehumidifier designs and design software ill-suited to HDH desalination applications. In this work a numerical dehumidifier model is developed and validated against experimental data. The model uses a logarithmic mass transfer driving force and an accurate Lewis number. The heat exchanger is subdivided into many cells for high accuracy. The Ackermann correction takes into account the effect of noncondensable gases on heat transfer during condensation. The influence of various heat exchanger design parameters is thoroughly investigated and suitable geometries are identified. Among others, the relationship between heat flow, pressure drop, and heat transfer area is shown. The thermal resistance of the condensate layer is n...

Pool Boiling Heat Transfer in Diluted Water/Glycerol Binary Solutions

Abstract: Pool boiling heat transfer in water/glycerol binary solutions has been experimentally investigated on a horizontal rod heater. The experiments have been performed at various concentrations (zero to 35% mass glycerol) and heat fluxes up to 92 kW m−2 at atmospheric pressure. The experimental values of boiling heat transfer coefficient have been compared to main existing correlations. It has been shown that the various predictions are significantly inconsistent. Based on the high difference between relative volatilities of water and glycerol, a simple model has been proposed to predict the boiling heat transfer coefficient. The applicability of this model is limited to low concentrations of glycerol and medium/low heat fluxes; however, the predictions are accurate. The proposed model is anticipated to be extendable to other binary systems in which the vapor pressure of one constituent is considerably higher when compared to the other component.

Effects of Sand–Bentonite Backfill Materials on the Thermal Performance of Borehole Heat Exchangers

Abstract: This article presents a quantitative analysis of the effects of sand–bentonite backfill materials on the thermal performance of borehole heat exchangers (BHEs). Laboratory thermal probe tests were conducted to measure the thermal conductivity of sand–bentonite mixtures under different mixed ratios. Based on microscopic observations, the mechanism of bentonite affecting heat conduction between the sand grains was analyzed. Then field tests were carried out to compare the thermal performance of two double U-shaped BHEs with different backfill materials. Test results showed that the thermal conductivity of sand–bentonite mixtures first increased with increasing percentage of bentonite by dry mass, then reached a peak at the range from 10% to 12%, beyond which the thermal conductivity decreased quickly. For the BHE with an optimal sand–bentonite backfill material, the heat injection and heat extraction rate were enhanced on average by 31.1% and 22.2%, respectively, compared with the case with a common sand–cl...

Enhanced Transient Heat Transfer From Arrays of Jets Impinging on a Moving Plate

Abstract: This article addresses the numerical analysis of single and multiple circular jets impinging perpendicularly on a flat plate for heating and cooling purposes. Computational fluid dynamics (CFD) is used to evaluate heat transfer calculations for different configurations and different flow boundary conditions. The commercial CFD package FLUENT is employed with various turbulence models. Results for a single jet are validated against experimental data. The SST k − ω turbulence model is compared with the elliptic V2F model, and both were validated against experimental data. Results were obtained for a range of jet Reynolds numbers and jet-to-target distances. Optimization results for the single jet case are validated against experimental data. The SST k − ω and V2F turbulence models succeeded with a reasonable accuracy (within 20% error) in reproducing experimental results. The heat transfer rates from the use of multijet configurations are discussed in the article. Transient heat transfer between multiple je...

Boiling Heat Transfer of Multiple Impinging Jets on a Hot Moving Plate

Abstract: In this research, boiling heat transfer on a hot moving plate caused by multiple impinging water jets in multiple jet rows is studied. An inverse heat conduction code is developed to analyze the readings of thermocouples that are implemented inside the plate in order to find the surface values of temperature and heat flux. Effects of nozzle stagger, plate velocity, and jet line spacing are studied. Nozzle stagger is found to affect the uniformity of heat transfer across the width of the plate. Jet line spacing can affect the heat transfer between two adjacent jet rows. Plate speed is important only in the higher entry temperatures and in the impingement zone.

Numerical Analysis of Jet Impingement Heat Transfer at High Jet Reynolds Number and Large Temperature Difference

Abstract: Jet impingement heat transfer from a round gas jet to a flat wall was investigated numerically for a ratio of 2 between the jet inlet to wall distance and the jet inlet diameter. The influence of turbulence intensity at the jet inlet and choice of turbulence model on the wall heat transfer was investigated at a jet Reynolds number of 1.66 × 105 and a temperature difference between jet inlet and wall of 1600 K. The focus was on the convective heat transfer contribution as thermal radiation was not included in the investigation. A considerable influence of the turbulence intensity at the jet inlet was observed in the stagnation region, where the wall heat flux increased by a factor of almost 3 when increasing the turbulence intensity from 1.5% to 10%. The choice of turbulence model also influenced the heat transfer predictions significantly, especially in the stagnation region, where differences of up to about 100% were observed. Furthermore, the variation in stagnation point heat transfer was examined for ...

Hydrogen Production by Thermocatalytic Methane Decomposition

Abstract: This article reports on the use of mesoporous carbons as catalysts for hydrogen production by thermocatalytic decomposition of methane. The prepared ordered mesoporous carbons (OMCs) and commercial carbon materials (including disordered microporous carbon, mesoporous carbon, and carbon nanotube) were tested for their catalytic activities for methane decomposition in a fixed-bed reactor. Characterizations by different techniques including gas adsorption, x-ray photoelectron spectroscopy, and transmission electron microscopy were carried out for the pristine and used catalysts. Results showed that the initial activity was related to the chemical structure of the catalysts such as defects, while the long-term activity was related to the physical characteristics such as the BET surface area and pore volume. Unlike disordered carbons, OMCs with relatively larger uniform pores could maintain a steady catalytic activity for a longer time, followed by a sharp activity decline due to the blockage of most of the po...

Liquid–Liquid Test Reactions to Characterize Two-Phase Mixing in Microchannels

Abstract: Multiphase flow is often found in chemical engineering, food processing, or analytical devices. First contacting and droplet generation as well as coalescence and redispersion are important for the flow characteristics. In all processes, the channel geometry, fluid properties, and flow velocity determine the flow regime, droplet size, and interfacial area. The hydrolysis of alkyl acetates in organic phase with sodium hydroxide NaOH in the aqueous phase is investigated as flexible test reaction for mass transfer and interfacial area determination. The alkyl group is chosen from ethyl, isopropyl, or n-butyl, which differ in water solubility, diffusivity, and rate constant, for adequate design of the characteristic time for mass transfer. The consumption of NaOH is used for calculation of specific area and related mass transfer coefficient. Different channel geometries are characterized and design considerations are derived.

The Influence of Horizontal Longitudinal Vibrations and the Condensation Section Temperature on the Heat Transfer Performance of a Heat Pipe

Abstract: This study is primarily focused on the influence of horizontal longitudinal vibrations and the condensation section temperature on the heat transfer performance of a grooved cylindrical copper heat pipe, with a length of 600 mm and an outer diameter of 8 mm. Longitudinal vibrations with frequencies of 3, 4, 5, 6, and 9 Hz and amplitudes of 2.8, 5, 10, 15, 20, and 25 mm, which would give accelerations in the range of 0.1–1.01 g, were experimentally tested. The condensation section temperature was set at 20, 30, or 40°C. A heating jacket and a cooling sleeve were installed at the evaporation and condensation sections of the test cell to mimic a constant heat flux and a constant temperature boundary, respectively. When the heat pipe started to vibrate horizontally in the longitudinal direction, this vibration caused an increase in the heat transfer of the heat pipe that was directly proportional to the input vibration energy below 500 mm2 Hz2. When the value of the vibration energy exceeded this value, the h...

Analysis of Secondary Flow Instability and Forced Convection in Fluid Flow Through Rectangular and Elliptical Curved Ducts

Abstract: This article examines the unique fluid flow characteristics and associated forced convection in curved ducts where the flow behavior is typified by counterrotating secondary flow vortices arising from the centrifugal forces due to flow curvature. For laminar developing fluid flow through curved heated ducts, the study formulates a novel three-dimensional computational fluid dynamics model based on vortex structures (or helicity). The fluid and thermal characteristics are examined using the helicity contours in duct cross sections for a range of flow rates, wall heat fluxes, and duct aspect ratios at selected duct curvatures. Curved ducts of rectangular and elliptical cross section are analyzed to identify and compare the fundamental differences in flow characteristics for each duct type. The study also presents a new technique using dimensionless helicity for detecting the onset of hydrodynamic instability in curved ducts. Numerical predictions are validated with the available experimental data. It is obs...

Modeling Transient Heat Transfer in Small-Size Twin Pipes for End-User Connections to Low-Energy District Heating Networks

Abstract: The low-energy district heating concept has the potential of increasing the energy and exergy efficiencies of heat supply systems and of exploiting renewable energy, provided that technical solutions for its wide application can be developed and implemented. This article investigates the dynamic behavior of district heating branch pipes in low-temperature operation (supply temperature 50–55°C and return temperature 20–25°C). We looked at state-of-the-art district heating branch pipes, suitable for the connection of a typical single-family house to a substation equipped with a heat exchanger for domestic hot water preparation. Experimental measurements of the supply temperature profiles at the outlet of the pipe, that is, at the inlet to the substation, were compared with detailed simulations based on the finite-volume (FV) method. A programming code was developed to model these profiles, and this was validated against experimental measurements and compared to the results of an analytical formula and the F...