Driving Temperature Differences Research Articles

Effects of an Annular Baffle on Heat Transfer to an Immersed Coil Heat Exchanger in Thermally Stratified Tanks

Abstract The effect of a cylindrical baffle on heat transfer to an immersed heat exchanger is investigated in initially thermally stratified tanks. The heat exchanger is located in the annular region created by the baffle and the tank wall. Three different cases of initial thermal stratification are explored, and in each case, experiments are conducted with and without the baffle in the stratified tank and in a comparable isothermal tank with the same initial energy, enabling exploration of the role of the baffle in a stratified tank and the role of stratification in tanks with or without the baffle. The baffle maintains the high initial temperature of the upper zone of the stratified tank for 10–16 min, as cool plumes that form on the heat exchanger are confined to the annular baffle region until they exit at the bottom of the tank. Regardless of stratification, the baffle always improves heat transfer to the immersed heat exchanger. In the isothermal tanks, the baffle increases total energy extracted in the first 30 min of discharge by over 20%. In stratified tanks, the baffle increases total energy extracted in 30 min of discharge by 9–16%. Initially, improvement in heat transfer in stratified tanks is due to the higher driving temperature differences around the heat exchanger. Later, after all the water from the hot zone has entered and flowed through the baffle, the tank is basically isothermal, and velocity increases as the fluid temperature drops, maintaining rates of heat transfer higher than that in the tank without the baffle. Stratification improves heat transfer in tanks without a baffle because, by design, the driving temperature difference between the heat exchanger wall and the surrounding fluid is considerably higher. However, in tanks with the baffle, stratification has only a modest positive effect on heat transfer to the immersed heat exchanger.

Review and Analysis of Heat Transfer in Spacer-Filled Channels of Membrane Distillation Systems.

Membrane distillation (MD) is an attractive process for the concentration of seawater brines. Modelling and simulation of membrane distillation processes requires a better knowledge of the heat transfer coefficients in spacer-filled channels which are usually determined by applying empirical correlations for the Nusselt number. In this study, first, a comprehensive literature review on heat transfer correlations was conducted. It was found that the empirical correlations often used for MD simulation result in strongly varying Nusselt numbers that differ by up to an order of magnitude at low Reynolds numbers. Then, heat transfer in spacer-filled channels was investigated experimentally in a membrane distillation system using an aluminum plate instead of a flat-sheet membrane. Numerous tests were carried out with sodium chloride solutions in a wide range of salinities, between 1 g/kg and 95 g/kg, and temperatures, between 30 °C and 80 °C, yielding high heat transfer coefficients in a range of 1500 to 8300 W/(m2K) at relatively low Reynolds numbers, between 100 and 1500, clearly showing the influence of the spacers on heat transfer. A new empirical Nusselt correlation (Nu=0.158Re0.652Pr0.277) was derived which represents the experimental data with a deviation of 10% and is valid for 100<Re<1500 and 2<Pr<7. Computational fluid dynamics simulations were performed to analyze the variations of the fluid properties across the boundary layer due to temperature differences. The simulations showed only minor deviations of the heat transfer coefficients in the hot and cold fluid channels for small driving temperature differences.

Simulation of a pillow plate thermosiphon reboiler using an equivalent tube bundle geometry

Pillow plate thermosiphon reboilers are innovative heat exchangers due to their compactness, flexible design options as well as high temperature and pressure resistance. Furthermore, thanks to their design they stabilize the recirculation flow and thus extend the operating range. However, due to their special geometry, available simulation tools do not allow for a quantitative rating or sizing. In this study, the geometry of a pillow plate apparatus is converted to an equivalent shell and tube geometry. It is investigated whether experimentally obtained results in a pillow plate thermosiphon reboiler can be reproduced by using that geometric equivalent in two industrial heat exchanger simulation tools. Eight experimental cases with varied operating pressures, submergences, and driving temperature differences are studied and compared to simulation results based on the geometrical equivalent. Both simulation tools provide acceptable results with deviations between +15% and −30% compared to the experimental determined circulation and heat flow. Additionally, they illustrate a special feature of the pillow plate thermosiphon reboiler: the pinch point appears at a shorter axial length than in a typical shell and tube thermosiphon reboiler, providing a larger area for evaporation.

Simulation of Thermosiphon Reboilers Using Wire Matrix Inserts at Lower Operating Range

Heat transfer enhancement to enlarge the operating range enables a more efficient use of thermosiphon reboilers. For this purpose, the use of wire matrix inserts in thermosiphon reboilers was studied experimentally and via simulation. The boiling of water/glycerol as a wide boiling mixture with an elevated viscosity was investigated at a sub-atmospheric pressure of 0.2 bar abs and a submergence of 110%. With varied driving temperature differences, single-phase reboiler inlet velocities from 0.01 m·s-1 to 0.16 m · s-1 were induced coupled with heat fluxes ranging from 1 kW·m-2 to 22 kW·m-2. By using inserts, the reboiler showed significant improvement of fluid dynamics and thermal performance and could be operated at lower temperature differences. The simulation confirmed the advantageous usage of inserts. However, the heat transfer was underestimated possibly resulting from the neglected subcooled boiling and deficits in the flow boiling model for the wide-boiling mixture at low pressures.

Heat transfer and wetting behaviour of falling liquid films in inclined tubes with structured surfaces

This study examines wetting behaviour and evaporation heat transfer of a falling liquid film inside a circular tube carrying grooves at its internal surface. The experiments include five different structures (four helical, one longitudinal) and one smooth tube (Cu, d i ≈ 27 mm ) with counter-current flow of liquid (downwards) and vapour (upwards). Results are obtained for liquid propane films ( P r = 2 . 79 , R e smooth tube = 160 − 645 ) at different twisting angles and height of the grooves, tube inclination angles ( β = 0° − 20° from the vertical), mass flow rates (from 5 kg/h up to 20 kg/h) and driving temperature differences (2 K, 3 K and 4 K) mainly at a film inlet temperature of 293.15 K. For wetting behaviour investigations the liquid film is visually observed at the lower end of the tube. The experiments reveal that the internal structures used lead to a strong improvement in both wetting behaviour and heat transfer, even up to inclination angles of 20° . The amount of capillary transported liquid within the grooves coupled with groove geometry itself are the main impacts on wetting behaviour and will be discussed within this paper. • Helically twisted structures resulting better wetting and heat transfer. • Greater groove mass flow rate supportive for greater heat transfer. • High aspect ratios supportive for high heat transfer. • Driving temperature difference influences wetting and heat transfer. • Small inclination with small influence on heat transfer.

On the Individual Droplet Growth Modeling and Heat Transfer Analysis in Dropwise Condensation

The low convective coefficient at the condenser part of spreaders and vapor chambers due to film blanket blocking encourages utilizing dropwise condensation (DWC). Challenges exist in the experimental characterization of DWC, which includes dependence on numerous parameters and, more importantly, measurement difficulties due to low driving temperature differences. This highlights the necessity of accurate modeling of this complex process. The widely used macroscale modeling process of DWC, known as classical analytical modeling of DWC, typically combines state-of-the-art droplet size distribution model with a simplified shape-factor-based heat transfer analysis of a single droplet that contains major simplifications, such as conduction only through the bulk liquid, hemispheric droplet shape, and homogeneously distributed temperature over the entire droplet surface. Recent numerical approaches included the effect of the Marangoni convection and implanted realistic thermal boundary conditions on liquid–vapor interface and reported significant errors of classical modeling. Based on a novel dynamic numerical approach that incorporates surface tension, the Marangoni convection, and active mass transfer at the liquid–vapor interface, the droplet growth phenomenon has been modeled in this study. Notable differences of droplet growth and flow field have been observed resulted from dynamic growth modeling of the droplet as more than 70% heat transfer rate underestimation of quasi-steady modeling in 1-mm droplets with a contact angle of 150° is observed. The effect of shape change due to gravity on the heat and mass transfer analyses of individual droplets found to be negligible.

Use of Wire Matrix Inserts in Thermosiphon Reboilers in the Lower Operating Range

AbstractTurbulators are applied to increase the thermal efficiency of heat transfer units. The additional pressure drop can be challenging for the self‐circulation of thermosiphon reboilers. Thus, the effects of hiTRAN®‐wire matrix inserts for the boiling of water and a water/glycerol mixture were investigated herein, using the performance of a thermosiphon reboiler with plain tubes as a reference. The reboiler was operated at sub‐atmospheric pressures, small driving temperature differences, and under flooded conditions. Favorable and unfavorable operating conditions for using inserts were specified. Especially for the water/glycerol mixture, significant improvements of self‐circulation and heat transfer up to six times compared to the plain tube reference were observed, allowing an operation of thermosiphon reboilers at smaller driving temperature differences under sub‐atmospheric pressures.

Novel Adsorption Cycle for High-Efficiency Adsorption Heat Pumps and Chillers: Modeling and Simulation Results

A novel thermodynamic cycle for adsorption heat pumps and chillers is presented. It shows a significant improvement of the internal heat recovery between the adsorption and the desorption half cycle. A stratified thermal storage, which allows for a temperature-based extraction and insertion of storage fluid, is hydraulically coupled with a single adsorber. The benefit is an increased efficiency by reusing the released heat of adsorption for regeneration of the adsorber and by rendering possible low driving temperature differences. For investigating the second law of this cycle, a dynamic model is employed. The transient behavior of the system and the respective losses because of driving temperature differences at the heat exchangers and losses due to mixing within the storage and to the surroundings are depicted in this one-dimensional model. The model is suitable both for analyzing this advanced cycle as well as for comparisons with other cycles.

A heat driven elastocaloric cooling system

Elastocaloric cooling is based on the latent heat associated with phase transformation upon stress variations in shape memory materials, which has significant potential to reduce the environmental footprints for air-conditioning and refrigeration industry. However, current elastocaloric cooling prototypes are limited by their driver to refrigerant mass ratio of over 500. To overcome this bottleneck, a new cycle using high temperature shape memory alloy as the heat driven actuator is proposed, which can precisely match the force-displacement characteristics of the refrigerant super-elastic alloy, and thus could potentially reduce the driver to refrigerant mass ratio down to the magnitude of 1. Numerical simulations are carried out to study the transient characteristics of the new cycle and the impacts of the heat source temperature and transformation temperatures of the actuator alloy. These two groups of parameters are correlated as the thermodynamic driving temperature differences. Furthermore, the theoretical minimum driving temperature shows that a low grade heat source around 80 °C is sufficient, which opens a new path for better utilization of the renewable energy and the waste heat. In the end, the design of a demonstrator based on the proposed cycle is presented, which shows a new path to develop compact elastocaloric cooling prototypes.

Phase state and velocity measurements with high temporal and spatial resolution during melting of n-octadecane in a rectangular enclosure with two heated vertical sides

A novel validation experiment for the melting of a phase change material is presented. The goal is to measure phase state and velocities with high accuracy and resolution. The geometry and boundary conditions of the test section are the most generic found in latent heat storage systems: a phase change material is contained in a rectangular enclosure, where it is isothermally heated from two opposing vertical side walls. The enclosure has a height of 105 mm and a width and depth of 50 mm. The bottom, front and back sides are solid transparent walls and on top is a thin layer of air. Near-adiabatic boundary conditions are realized at the non-heated sides with a surrounding insulated air-filled chamber and an actively controlled trace heating system. In this study n-octadecane is used as the phase change material. During melting, the liquid phase fraction is measured with a shadowgraph technique and velocities due to natural convection in the liquid phase are measured with particle image velocimetry (PIV). Interior and boundary temperatures are measured with thermocouples to control and analyze boundary effects. A thorough error estimation is done for all the measured quantities. The main result is a comprehensive dataset of liquid phase fractions and velocities with high spatial and temporal resolutions. The liquid phase fraction is additionally measured for three different driving temperature differences and a scaling by dimensionless numbers is performed. This results in a correlation function for the liquid phase fraction that predicts similar melting processes and is valuable in system design and optimization.

Numerical analysis of heat exchanger designs for passive spent fuel pool cooling to ambient air

Passive cooling of spent fuel pools via natural two-phase convection of a fluid with low boiling point is a promising alternative to active cooling circuits as such a passive heat transfer system would still work in safety–critical situations, such as a station blackout. For ambient air as the ultimate heat sink the heat exchanger design plays a crucial role as driving temperature differences may be low. This paper outlines a numerical investigation on a finned oval tube bundle heat exchanger operated under natural air convection in a chimney. We studied the role of chimney geometry and heat exchanger fin geometry. With respect to the chimney we found that velocity, Nusselt number and heat transfer are enhanced by 161.3%, 31.7% and 62.5% respectively, if chimney height increases from 2 m to 16 m. With respect to the fin design we determined an optimal fin configuration with a fin height of 17 mm, fin spacing of 3 mm and fin thickness of 1.5 mm, which improves the heat transfer performance by 28.7%, the Nusselt number by 28.9% and the fin efficiency by 19.2% at a given temperature difference of 40 K. The final optimized finned tube bundle heat exchanger design achieves a volumetric heat transfer density of q‾vol=3.61kWmK.

Experimental, numerical and analytic study of unconstrained melting in a vertical cylinder with a focus on mushy region effects

The enthalpy-porosity method is widely used in solving solid-liquid phase change problems that involve convection in the melt; however the influence of the required mushy zone parameter on the melting process has been largely overlooked. In this paper, further investigation of the mushy zone parameter is presented. The enthalpy-porosity method is the default model in Fluent for melting simulations. A comprehensive discussion of previously reported mushy zone parameter values is presented with a comparison to numerical and experimental results. In this paper, based on experimental validations of melting times, it is found that mushy zone parameters can be optimized based on relevant driving temperature differences. And despite the fact that the model cannot capture bulk solid sinking behaviors, numerical solid sinking behaviors by Fluent are still widely reported in the literature. Explanations and supporting numerical analysis are given for this seeming contradiction. Finally, an analytic solution for unconstrained sinking is developed. With the introduction of a tuning parameter to modify the viscosity of the mushy region in the bottom liquid layer, good agreement between the analytical model and experimental results is achieved. A linear correlation for the tuning parameter based on driving temperature differences is given.

A new formulation for convection problems entailing multiple isothermal boundaries

Many convection problems entail more than two isothermal boundaries. In previous work, a resistor-network model was proposed for this class, multi-temperature convection problems. A technique dubbed dQdT was also developed to obtain the paired convective resistances that characterize the thermal network of a multi-temperature convection problem. In the present paper an extension of the Newton law of cooling is proposed as a general formulation of multi-temperature convection in terms of multiple driving temperature differences. Most notably, the proposed formulation eliminates the need for an effective temperature difference. The formulation is characterized by functionality coefficients which give the relation between a heat transfer rate and one of the temperature differences. These coefficients can be obtained using the dQdT technique. The new formulation and the application of the dQdT technique are demonstrated for classical three-temperature convection problems. The connection between the extended Newton formulation and the resistor-network model of multi-temperature convection is also discussed. It is shown that dQdT can be used to determine the applicability of the resistor-network model of convection.

A novel adsorption module with fiber heat exchangers: Performance analysis based on driving temperature differences

A main focus of recent R&D on adsorption modules for thermally driven heat pumps and chillers has been to enhance the volume specific power output while maintaining a reasonable coefficient of performance (COP).An adsorption module using a new type of heat exchanger based on aluminum sintered metal fiber structures brazed on flat fluid channels has been developed. The heat exchangers for adsorber/desorber and evaporator/condenser are identically constructed. The adsorption heat exchanger is coated with a silico-alumino phosphate (SAPO-34) by a partial support transformation direct crystallization (PST) [1]. Both components are placed in a vacuum tight housing using a valve-free configuration. Water is used as adsorptive.The experimental characterization of the module shows a high volume specific power (up to 82 W/litre module for cooling, 320 W/litre for heating). Although no heat is recovered between ad- and desorption cycle, a COP of almost 0.4 is reached for cooling and 1.4 for heating.Driving temperature differences are defined for the analysis of the heat exchanger performance. The evaporator/condenser shows extremely good performance with about 240 W/K specific evaporation power per litre of heat exchanger, while the adsorber is limiting the module performance.

Condensation of carbon dioxide in microchannels

Heat transfer coefficients and pressure drops during condensation of carbon dioxide (CO2) are measured in small quality increments in rectangular microchannels of 0.10⩽Dh⩽0.16mm and aspect ratios from 1 to 4. Channels are fabricated on a copper substrate by electroforming copper onto a mask patterned by X-ray lithography, and sealed by diffusion bonding. The test section is cooled by chilled water circulating at a high flow rate to ensure that the thermal resistance on the condensation side dominates. A conjugate heat transfer analysis, in conjunction with local pressure drop profiles allows driving temperature differences, heat transfer rates, and condensation heat transfer coefficients to be determined accurately. Condensation heat transfer coefficients and pressure drops are measured for G=400, 600 and 800kgm−2s−1, for 0<x<1, and saturation temperatures of 15, 20 and 25°C (Pr=0.69, 0.78 and 0.87). The data are used to evaluate the applicability of correlations developed for larger hydraulic diameters and different fluids for predicting condensation heat transfer and pressure drop of CO2.

Thermosiphon Reboilers with Enhanced Tubes

AbstractReliable heat exchangers capable of transferring heat at low driving temperature differences are key to energy efficient processes. In this paper, heat transfer and fluid dynamics of hydrophobic nanoparticle (HBNP) coated tubes are investigated in a steam‐heated single‐tube thermosiphon reboiler and compared to experimental results of standard stainless steel tubes. In comparison to the bare tube, the coated tube showed higher heat transfer and circulation rates as well as an improved stability of the circulating flow. The stability of the coating was verified in a long‐term stability test.

Qualitative investigation of the flow behaviour during falling film evaporation of a dairy product

Falling film evaporation is an important technology in the dairy industry for producing powders. In this paper, flow details of liquid falling films have qualitatively been investigated using a pilot evaporator, and in particular using a high-speed camera. Variations with different dry solids contents, flow rates and driving temperature differences were investigated. The flow characteristics were seen to be considerably affected by all three variables. Two of the main observations were the formation of bubbles under evaporative conditions and that the flow, bubble formation and evaporative heat transfer coefficient was observed to be heat flux dependent.

MEASUREMENT AND PREDICTION OF VAPOR-SPACE CONDENSATION OF REFRIGERANTS ON TRAPEZOIDAL-FINNED AND TURBO-C GEOMETRIES

This paper reports vapor-space condensation heat transfer measurements for R123, R134a, and R245fa for the integral trapezoidal fin, and the Turbo-CII geometries on vertical plates. The data consisted of heat flux and wall temperature difference measurements. Condensation heat transfer measurements on a smooth plate agreed well with both measurements and predictions from the literature. Overall, the heat transfer performance of the three refrigerants on the trapezoidal fin was within approximately 8 kW/m of one another. Similarly, the condensation heat flux for R134a and R245fa on the Turbo-CII was within approximately 18 kW/m of each other, while the heat flux of R123 on the Turbo-CII was between 10 and 80 kW/m less than that of R245fa. An existing finned tube condensation model was modified to be expressed in terms of the gradient of the condensate curvature with respect to the length of the liquid–vapor interface. Curvature gradients for the two surfaces were developed that, when substituted into the modified model, predicted the present measured driving temperature differences for the trapezoidal fin and the Turbo-CII to within 0.4 and 1.2 K, respectively, for all measurements except for R123 on the Turbo-CII surface. With the aid of the curvature gradients, simple models were developed to predict the performance of the trapezoidal, low-finned tube, and the Turbo-C tube. The heat flux to the low-finned tube and the Turbo-C tube were predicted to within 10% and 15%, respectively, of the measured values from the literature for four different fluids.

NANOTECHNOLOGY FOR ADVANCED NUCLEAR THERMAL-HYDRAULICS AND SAFETY: BOILING AND CONDENSATION

A variety of Generation III/III+ water-cooled reactor designs featuring enhanced safety and improved economics are being proposed by nuclear power industries around the world in efforts to solve the future energy supply shortfall. Thermal-hydraulics is recognized as a key scientific subject in the development of innovative reactor systems. Phase change by boiling and condensation in the reverse process is a highly efficient heat transport mechanism that accommodates large heat fluxes with relatively small driving temperature differences. This mode of heat transfer is encountered in a wide spectrum of nuclear systems, and thus it is necessary to determine the thermal limit of water-cooled nuclear energy conversion in terms of economic and safety. Such applications are being advanced with the introduction of new technologies such as nanotechnology. Here, we investigated newly-introduced nanotechnologies relevant to boiling and condensation in general engineering applications. We also evaluated the potential linkage between such new advancements and nuclear applications in terms of advanced nuclear thermal-hydraulics.

Representative Results for Condensation Measurements at Hydraulic Diameters ∼100 Microns

Condensation pressure drops and heat transfer coefficients for refrigerant R134a flowing through rectangular microchannels with hydraulic diameters ranging from 100 μm to 200 μm are measured in small quality increments. The channels are fabricated on a copper substrate by electroforming copper onto a mask patterned by X-ray lithography and sealed by diffusion bonding. Subcooled liquid is electrically heated to the desired quality, followed by condensation in the test section. Downstream of the test section, another electric heater is used to heat the refrigerant to a superheated state. Energy balances on the preheaters and postheaters establish the refrigerant inlet and outlet states at the test section. Water at a high flow rate serves as the test-section coolant to ensure that the condensation side presents the governing thermal resistance. Heat transfer coefficients are measured for mass fluxes ranging from 200 kg/m2 s to 800 kg/m2 s for 0&lt; quality &lt;1 at several different saturation temperatures. Conjugate heat transfer analyses are conducted in conjunction with local pressure drop profiles to obtain accurate driving temperature differences and heat transfer coefficients. The effects of quality, mass flux, and saturation temperature on condensation pressure drops and heat transfer coefficients are illustrated through these experiments.