Dimensionless Exergy Research Articles

Comparison between MIX number and dimensionless exergy as performance indicators in a commercial stratified storage for future optimization of solar field operations

ABSTRACT The growing need for renewable energy sources has highlighted the importance of technologies that can bridge the generation-demand gap in the energy system. Thermal Energy Storage (TES) can help to smooth out peaks in energy demand, thereby reducing waste resulting from excess capacity during off-peak periods. Due to their widespread use, significant effort has been dedicated to optimizing the control logic of TES components to maximize the harvested-to-stored energy ratio. The aim of this study is to compare MIX number and dimensionless exergy, two parameters commonly used to identify the storage’s ability to generate and maintain optimal temperature stratification. The investigation is conducted by comparing the parameter response to the thermo-fluid dynamic variations inside the store under several inlet temperatures and flow rates, using a two-dimensional CFD model validated on experimental data collected from an operating commercial stratified tank. The sensitivity of the two performance indicators in detecting stratification losses that are due to different charging conditions is compared, to better understand the applicability of the latter as control parameters during operational phases. Results show that the derivatives of such indicators are less sensible to the tank starting conditions and can be more robust indicators during the charging phase.

A state-space heating and cooling model for thermal stratification in horizontally configured electric hot water storage cylinders

Domestic water heating significantly impacts global energy demand, with electric hot water storage devices notably contributing to grid load. Efficient management of these widespread devices is contingent on a deep understanding of the internal thermal stratification occurring naturally during heating and cooling phases. While extensive research exists for vertical water heaters, horizontal heaters, more challenging to model, have been comparatively overlooked in scholarly research. This paper introduces a state-space model developed using measured datasets, aimed at accurately describing temperature distributions inside a horizontally-configured electric hot water storage cylinder during heating, thermal diffusion, and cooling stages. The model’s effectiveness was validated through various evaluation metrics and separate, unseen evaluation datasets. It achieved an accuracy of 90.6% for the dimensionless stratification factor, 94.6% for the dimensionless exergy number, and reported Root Mean Square Errors (RMSEs) of 150kJ for energy, 11kJ for exergy, and 0.8°C for temperature.

Experimental data analysis and dimensionless exergy levels in a commercial stratified thermal storage

The continuously rising demand for renewable energy sources within the energy system has highlighted the need for technologies capable of mitigating the impact of renewable energy intermittency and addressing the generation-demand mismatch in the system. Thermal energy storage (TES) emerges as a versatile storage medium with a wide range of applications, spanning from solar energy utilization and power peak shaving to the storage of industrial waste heat. In this study, data collected from an operating commercial stratified tank are used to validate a 2-D axisymmetric CFD model. Temperature profiles are collected throughout one month with a one-minute refresh rate. The model replicating the tank is generated in COMSOL Multiphysics® and validated by emulating the registered charging phases of the real storage. The model is then employed to optimize the stratification capability of the tank, by varying the logics applied to pinpoint optimal values of both inlet water temperature and velocity. The study aims to maximize the exergetic efficiency of the system using dimensionless exergy, a parameter often utilized in literature to identify the ability of the storage to generate and preserve optimal temperature stratification. Finally, the experimental dimensionless exergy has been evaluated for the aforementioned temperature profiles.

Effect of initial temperature of water in a solar hot water storage tank on the thermal stratification under the discharging mode

Thermal stratification in a storage tank is an important factor affecting hot water output. There are many factors affecting thermal stratification, and the initial temperature in the tank is a notable one. In this paper, a typical solar hot water storage (HWS) tank, which is widely used in practical applications, has been selected to study numerically the effect of the initial temperature on the thermal stratification under the discharging mode. The flow mechanism, temperature distribution, thermal stratification, and thermal mixing mechanism in the tank are analyzed with the numerically simulated results for five initial temperatures in the tank. In addition to the qualitative observation, a quantitative analysis of the effect of the initial temperature in the tank has also been conducted in terms of several key dimensionless parameters which quantify the extent of the thermal stratification in the tank. The results show that the dimensionless exergy decreases with the dimensionless time. When the dimensionless time increases from 0.1 to 0.8, the dimensionless exergy decreases by nearly 0.4. Additionally, a higher initial temperature leads to a smaller extent of the initial mixing and a larger output rate of hot water. When the initial temperature in the tank increases from 313 K to 353 K, the output rate of hot water increases by nearly 11%. Moreover, the extent of the thermal stratification increases almost linearly with the initial temperature in the tank. When the initial temperature in the tank increases from 313 K to 353 K, a 17% increase in the thermal stratification has been achieved indicating that the initial temperature in the tank has a significant effect on the thermal stratification, which will be insightful for the better design of solar hot water systems.

HOW THE ESTIMATION OF ENTROPY GENERATION AND EXERGY LOSS OF HYBRID NANOFLUIDS GOVERNS THE THERMAL PERFORMANCE OF HEAT EXCHANGER

The present study reports heat-transfer performance, exergy analysis, entropy generation, and pressure drop of shell and helically coiled heat exchanger (SHCHE) with Al<sub>2</sub>O<sub>3</sub>-CuO/water hybrid nanofluid (HYNF) as a working fluid. Helical coil is made of copper material with 54 turns and pitch ratio is 31.35 mm. Hot oil streams at the shell with 75° C, and the working fluid streams at the helical coil with 30° C. The volume fraction of the nanoparticles is considered as 0.1 vol.%. Reynolds number of the oil is fixed as 900 and the Reynolds number of the working fluid varies from 6000 to 15,000. The numerical code is validated with the earlier experimental work. Highest thermal performance is obtained by using 0.1 vol.% HYNF than nanofluids and base fluid. Role of mass flow rate, and Reynolds number on heat-transfer rate, effectiveness, total entropy generation, exergetic efficiency, exergy loss, and dimensionless exergy loss are investigated. An ~ 20% increase in Nusselt number and ~ 48% increment in exergetic efficiency are noted with the usage of HYNF. Entropy generation of SHCHE is lower by adding nanoparticles. This study enables the readers to understand the irreversibility of heat transfer in shell and helically coiled heat exchanger.

Heat Transfer and Exergy Analysis of a Shell and Tube Heat Exchanger using PGW based ZnO Nanofluids

In this experimental work, ZnO nanoparticles were synthesized using the chemical precipitation method, and the nanoparticle structure and morphology were characterized through XRD and SEM. Heat transfer and exergetic characteristics were then studied in a shell and tube heat exchanger using PGW-based ZnO nanofluids varying nanoparticle volume concentration and nanofluid (shell side) flow rate at 6, 8, 10 and 12 litres/min. The hot water flow rate was fixed at 12 litres/min. The experimental results show that the heat transfer rate was improved by increasing the nanoparticle concentration and nanofluid flow rate. When the nanoparticle volume concentration was 0.3 per cent, the maximum enhancement of heat transfer rate and average heat transfer coefficient using ZnO nanofluids were 35.9 per cent and 40.2 per cent, respectively, in comparison to the base fluid. Exergy loss and dimensionless exergy loss both increased with nanofluid flow rate and dropped substantially with increased nanoparticle volume concentrations. The average increment of exergetic effectiveness at three different nanoparticle volume concentration (0.1%, 0.2%, and 0.3%) are 10.68%, 23.64%, and 31.23% respectively. The highest exergetic sustainability index (0.41) and lowest environmental impact factor (2.42) were observed when the nanoparticle concentration was 0.3% with the nanofluid flow rate of 6 litres/min.

Experimental study on entropic behavior of smooth pipe due to the simultaneous usage of spring turbulator and air bubble injection

Numerous investigations were conducted to enhance heat transfer in smooth tubes. They have used both active and passive methods. However, very rare investigations have reported the simultaneous usage of active and passive methods. Furthermore, the investigations on entropic behaviors are very rare too. Through present paper, effect of air bubble injection (as active method) together with spring wire turbulator (as the passive method) is probed. The pipe was subjected to a constant heat flux. Spring pitch and fluids flow rates were variable parameters of this study. Four flow levels of 2, 3, 4, and 5 were intended for water and air. The studied parameters were Nusselt number, cost-performance coefficient, and dimensionless exergy destruction. The flow patterns were also assessed. To calculate the Nusselt number and exergy destruction, ambient temperature, pipe surface temperature, water inlet, and outlet temperatures were measured. Results showed that reducing the induced rotational flow by the pitch, affects two-phase flow and leads significant changes flow pattern and two-phase flow motion model. Air bubble injection increased Nu by 40% and dimensionless exergy destruction by 36%.

EXPERIMENTAL STUDY OF HEAT TRANFER AUGMENTATION USING AIR BUBBLE INJECTION AND (Al2O3 /WATER) NANOFLUID FLOW IN DOUBLE PIPE HEAT EXCHANGERS

In the present work, an experimental study on how to increase the heat transfer coefficient (HTC) in double pipe heat exchanger (DPHE) use of a variety of Al2O3 Nano-dispersion concentrations mixed in water as base fluid with air bubble injection for counter flow arrangement under turbulent flow conditions with (Re) Reynold number range from (6000 t0 45000) . The thermal performance of (DPHE) has been enhanced with the use of two techniques. The first, is represented by adding nanoparticles to hot water (inner pipe) raising the (HTC) inside the inner tube. Increase the volume concentration cause increase in the viscosity of the nanofluid leading to increase in friction factor .Secondly is represented by Air bubble injection in outer pipe with cold water to enhance the (HTC). The mobility of air bubbles inside the water from down to up by the force of the buoyancy, and the movement of these air bubbles results in significant mixture and turbulence within the water. The variations of number of thermal units (NTU), exergy loss, dimensionless exergy and (Nu) are evaluated. The investigated parameters were cold water volume flow rates (8, 10, 12 and14) l/min, flow in outer tube. Also, three different volume flow rates of air (12, 16 and 20) l/min mixed with water in outer tube. The volume flow rates of hot water remains constant at (8 l/min) flow in inner pipe with three volumetric concentrations of given nanofluid. The results showed that the air bubble injection throughout the tube gave maximum enhancement in heat transfer characteristics followed by the no air bubble injection. Since the enhancement in heat transfer characteristics varies linearly with the volumetric concentration of Nanofluids, Nanofluids with 0.3% of Al2O3 nanoparticles gave more enhancements in (HTC) than the case without nanofluid. The Nusselt number increased about (8% - 45%).

Convection Heat Transfer Analysis with Flow Resistance for Mini-Helically Coiled Tubes at Supercritical Pressures Experimentally

This paper analyzes the effect of fluid flow characteristics on the convection heat transfer for mini-helically coiled tubes (HCT) using supercritical carbon dioxide (CO2) as a natural refrigerant. Two experimental cases have studied in this work for mini-helically coiled tubes at different diameters with different coil pitches for analyzing the convection heat transfer with flow resistance. In the first case, the inner tube diameter, coil diameter and coil pitch were 5 mm, 200 mm and 10 mm respectively, while 10 mm, 100 mm and 5 mm were for the second case. Moreover, this work has also investigated the influence of frictional pressure drop, heat flux, friction factor and mass flux on dimensionless exergy destruction. The work environments were 300-500 K as an inlet temperatures range, 200-2000 Kg / (m2. s) as a mass heat fluxes range, 50,000-500,000 as a Reynolds number (Re) range and 50-200 Kw/m2 as an inner heat fluxes range. As a result, a large effect has been observed for dimensionless exergy destruction compared with the flow friction of CO2 which induced by heat transfer irreversibility. On the other point of view, a good sensitivity of optimal Re with the tube dimeter and mass flux also noticed compared with the heat flux. At a suitable range for Re, smallest and best exergy destruction also noticed for the tube diameters. A correlation has for the optimal Reynolds number as function of main dimensionless parameters related to wall heat flux, mass flux, fluid properties and geometric dimensions is proposed. Characteristics of the fluid flow had influenced significantly by mass and heat fluxes. In the future, the collected experimental data can be employed in order to design and improve the refrigeration conditioning performance for exchangers and other systems such as heat pumps.

Performance and exergy analysis of different perforated rib designs of triple tubes heat exchanger employing hybrid nanofluids

In the current investigation, heat, flow characteristics, and exergy analysis of hybrid and single nanoparticles-based water passing through a triple ribbed tube heat exchanger (TRTHE) under several design and operation parameters are studied. A computational fluid dynamics (CFD) modeling is carried out using ANSYS-FLUENT 19 code while the validation criteria are accomplished with experimental data and empirical correlations in the literature. The model technique of 3D re-normalization group (RNG)/k–ϵ with enhanced wall treatment is chosen considering the conjugate heat transfer between the fluids. A user-defined function is utilized to specify the thermophysical properties of nanofluids and water based on the operating fluid temperature. The design and operating nanofluid velocity and temperature of inner, intermediate, and outer tubes are 0.7 m/s at 10 °C, 0.05–0.13 m/s at 70 °C, and 0.05 m/s at 18 °C, respectively. The ribs under consideration are trapezoidal, semi-circular, semi-elliptical, triangular, rectangular, and square-shaped and the tested perforated louver widths are 0.5, 1, 1.5, and 2 mm. Three hybrid combinations of nanomaterials (oxide/oxide, oxide/carbide, oxide/carbon nanotube) by constant volume percent of 50/50% are employed with water as a base fluid under a constant volume concentration of 0.1%. Four different kinds of nanoparticles are chosen as Al2O3, MgO, SiC, and MWCNT in order to perform the present study.The obtained findings manifested that the hybrid nanofluid of Al2O3+MWCNT/H2O achieves a higher heat transfer rate compared with single nanofluids. A lower rib louver width with semi-circular rib geometry and staggered rib arrangement leads to an increase in the Nusselt number and effectiveness of the TRTHE. The use of semi-circular rib shape produces a rise of dimensionless exergy loss rate by about 5.4–21.6% more than other rib shapes.

Thermal stratification in a solar hot water storage tank with mantle heat exchanger

Solar hot water storage tanks with mantle heat exchangers are widely used in balcony wall-mounted solar heating system in China. The thermal behaviour of a novelty mantle solar water tank with vertical inlet and outlet water tubes has been investigated both experimentally and numerically. Dimensionless parameter can well characterize the stratification of water and thermocline dynamics in the water tank. In the discharging mode, as the dimensionless time increases, the thermocline thickness gradually increases, and the mixing in the water tank intensifies. The dimensionless exergy gradually decreases with the increase of the dimensionless time.

Second law efficiency analysis of air injection into inner tube of double tube heat exchanger

At present study, thermal performance of a double tube heat exchanger due to stream of air/water two phase flow through inner tube is experimentally studied. Air and hot water were mixed in a T-junction outside heat exchanger and then were followed into inner tube of heat exchanger. Told water flow rate was kept constant and was equal to 2 lit/min. For hot water flow rate, four different flow rates of 3, 4, 5 and 6 lit/min was considered. Also inlet temperature of cold and hot water streams were almost constant and were within range of 17–19 °C and 48–50 °C. Also, for air flow rate five different flow rates of 1, 2, 3, 4 and 5 lit/min were considered. Volume fraction was within the range of 0.14 and 0.62. Obtained results were analyzed based on several energetic and exergitic parameters including pressure drop, effectiveness, Number of Transfer Units, heat transfer coefficient, dimensionless exergy loss and Witte-Shamsundar efficiency factor. Results presented an increment of 33% and 38% in heat transfer coefficient and Number of Transfer Units, respectively. Maximum value of Witte-Shamsundar efficiency factor was found to be 0.973 and was related to Volume fraction of 0.57 and has occurred at counter flow.

Investigation of the effect of the finned coiled wire insert on the heat transfer intensification of circular tube: Energy and exergy analysis

In the current investigation, CFD (computational fluid dynamics) code is applied for the three-dimensional simulation of fluid flow and turbulent transferred thermal energy through circular duct with coiled finned wire inserts. The realizable K-ε model will be used for the obtaining the influence of the turbulence. The validation of the model is performed by comparing the numerical Nusselt number with the experimental one. The impact of the various operating parameters i.e. Prandtl and Reynolds numbers and geometric variables i.e. fin height and thickness, coil pitch and wire diameter on the dimensionless exergy loss, thermal efficiency and the dimensionless working power will be obtained. Furthermore, correlations will be suggested to estimate these objective variables as functions of the geometric and operating variables. A new multi-objective optimization way is introduced and applied to obtain the optimal and critical values or range of the input variables in the studied range. Results also show if the Reynolds number increases by 100 % in the study range, the thermal efficiency, the dimensionless exergy loss and dimensional working power reduces by 15.9 %, 9.4 % and 5.9 %, respectively. Furthermore, with doubling pitch, they reduce 17.2 %, 14.9 % and 48.9 %, respectively.

Exergy analysis of Shell and helical coil heat exchanger and design optimization

In this study, experimental investigations and exergy analysis on shell and helically coiled tube heat exchanger are carried out for free convection heat transfer. The measured data are totally optimised utilizing thermodynamics rules in which exergy study is performed to investigate the thermal performance of the helical system under different operating conditions. The experimental set-up of apparatus are designed and made for cold water and hot water as a working fluid of both the shell side and helical coil side, respectively. The effects of several parameters such as geometry and operational conditions on the exergy destruction and dimensionless exergy destruction are investigated. The counter flow direction is considered under the steady state flow condition, and the critical Reynolds number was more than 4000 in this study. The main objective of this work was to clarify the effect of the volume flow rates and inlet temperatures of hot water and cold water in the shell and helical coil on exergy efficiency and pressure drop. Results showed that the exergy destruction and dimensionless exergy destruction decrease with the increase of coil pitch and Dean number. In contrast, the exergy destruction and dimensionless exergy destruction are obviously increased with the hot water flow rates or cold water flow rates. These exergy characteristics are also augmented with the values of hot water inlet temperatures and cold water inlet temperatures. The pressure drop is considerably increased with the increase of Dean number and reduced with the increase of coil diameter. While, the exergy efficiency steadily increases with the decrease of the cold water flow rates and with the increase of Dean number.

Experimental investigation of heat transfer and exergy loss in heat exchanger with air bubble injection technique

The main aim of this study is to evaluate thermal performance and exergy analysis of a shell-and-tube heat exchanger with a new technique called air bubble injection. The study has been carried out with different parameters such as flow rate, fluid inlet temperature, and different air injection techniques. The air has been injected at different locations such as the inlet of pipe, throughout the pipe, and in the outer pipe of the heat exchanger. Based on the results, the performance of the heat exchanger enhances with an increase in the flow rate and the fluid inlet temperature. The exergy loss and dimensionless exergy loss increase with a rise in the flow rate. The maximum and dimensionless exergy losses are obtained at a maximum flow rate of 3.5 l min−1. With the air bubble injection in the heat exchanger, it has been observed that the temperature difference increases, which leads to an increase in the exergy loss. The injecting air bubbles throughout the tube section shows that minimum dimensionless exergy is 27.49% concerning no air injection.

A comprehensive study of the second law of thermodynamics for a rotary regenerative heat exchanger with two types of simple and anodized aluminum sheets: an experimental study

ABSTRACT Rotary regenerative heat exchangers are commonly used in the industry. However, the corrosion of sheets that are in contact with the operating fluid is considered as the disadvantage of these heat exchangers. This causes a reduction in the lifespan of this kind of heat exchanger. The object of the present study is to evaluate and compare the exergy loss and the second law efficiency of a rotary regenerative heat exchanger, which is made from two types of simple aluminum sheets and anodized aluminum sheets are. Anodized sheets are used to enhance the corrosion resistance of the regenerative heat exchanger. The results indicated that the lower exergy loss and the dimensionless exergy loss with anodized sheets about 14–24% and 15–20% compared to the heat exchanger with simple sheets, respectively. Besides, the effects of rotational speed of the matrix, inlet temperature of hot flow, and the flow rates of hot and cold flows on the rotary regenerative heat exchanger are evaluated. Finally, some empirical correlations have been developed based on the obtained data and curve fitting process.

Heat transfer optimization through new form of pin type of finned tube heat exchangers using the exergy and energy analysis

Abstract In the present work, computational fluid dynamics method is used to simulate the three dimensional turbulent thermal energy transfer and fluid flow through the new types of the finned tube heat exchangers. The realizable turbulent kinetic energy - turbulent kinetic energy dissipation model is selected as the turbulence model because it has the best results compared to the other turbulence models with the experimental data. After the simulation the energy and exergy analysis are performed and a multi-objective model is defined to obtain the optimal and critical values of the operational and geometrical parameters. Furthermore, equations are suggested for estimation the three objective parameters i.e. thermal effectiveness, dimensionless exergy loss and dimensionless work. Results also indicate, the dimensionless exergy loss is optimum at Re=8333 and Pr=10.63. Furthermore, it was found, the optimum value for the transverse pitch is equal to 3.52, when the objective parameters are the thermal effectiveness and the dimensionless exergy loss.

Thermal analysis of sinter vertical cooler based on waste heat recovery

Sinter vertical cooler, as a new type of sinter waste heat efficient recycling device, is facing great potential and rare opportunities for development. The structural and operating parameters greatly affect the subsequent utilization of waste heat. In the present work, the 3-D model has been improved, and the novelty is that a convection term is added to the energy equation of the solid phase to achieve the steady-state heat transfer condition in gas solid state. The reliability of the mathematical model was verified by the experimental data. Four main parameters were investigated based on exergy, energy consumption and dimensionless exergy consumption. The results show that, when the other parameters are constant, the optimal cooling gas mass flow rate is 85 kg/s based on the maximum exergy. The greatest impact on dimensionless exergy consumption is cooling section diameter, followed by cooling gas mass flow rate, cooling section height and sinter mass flow rate. The structural and operating parameters have a monotonous effect not only on the exergy but also the energy consumption. These results should be helpful for understanding the flow and heat transfer process in the sinter vertical cooler for waste heat recovery engineering.

Energy and exergy analysis of compact automotive air conditioning (AAC) system

Automotive Air conditioning (AAC) is a unit that uses a high measure of energy from a car total engine power. In equatorial climate country such as Malaysia, high usage of AAC is inevitable due to hot, humid and rainy weather throughout the year. An understanding about the energy and exergy losses is essential to find the potential improvement to maximise the efficiency in an AAC system. The main objective of this study is to study the performance of energy and the exergy of a compact automotive air conditioning system. This cycle uses R134a and PAG lubricant as the working fluid. The different ranges of initial refrigerant charge and compressor speed have been tested on the AAC to evaluate the effect of different major thermodynamic parameters in performance. A theoretical model is developed to work out the thermodynamic parameters such as coefficient of performance, exergy destruction ratio, component efficiency defect as well as the dimensionless exergy balance for the AAC system components. The results of this study have shown that most of the energy has been destructed in evaporator part. In order to maximize the efficiency and performance of AAC system, further optimization needs to be done in order to improve the evaporator component.

The effect of geometrical characteristics of wavy strip turbulator and thermodynamic properties of fluid on exergy loss and heat transfer in a tube in tube heat exchanger

ABSTRACTThe present work conducts an experimental investigation into the influence of flow, thermodynamic and geometrical characteristics of the wavy strip on exergy loss and dimensionless exergy loss in a tube in tube heat exchanger. The working fluid is water with hot water passing the inner tube and cold water passing annulus. Wavy strips with four different angles and three widths were investigated experimentally. The result of exergy loss and dimensionless exergy loss for various conditions is presented and on the basis of curve fitting, three empirical correlations are suggested to predict dimensionless exergy loss in a double tube heat exchanger.