Constant Inlet Temperature Research Articles

Comparison of heat transfer rate in copper and aluminium finned tube heat exchanger

This paper provides an overview on factors that affects the cooling rate capacity and some materials are studied. Properties compared in this paper include fluid flow rate, air velocity, and temperature limitations. Basics ideas of difference of materials used in Heat Exchanger are studied. For the objective of understanding the changes in parameters and behavior of Heat Exchanger and experimental setup layout was formed. The setup of Heat Exchanger was fabricated and tested for its working. Variation of heat exchange from water to air was studied. The water was flowing through tube and air was flowing through air duct by blower. Exchange of heat from hot water to cold air took place. This experiment was conducted for both Aluminium and Copper finned tube Heat Exchanger. Heat gained by air at various mass flow rate at constant water inlet temperature was noted for both Copper and Aluminium finned tube Heat Exchanger. From the graphs obtained it is concluded that heat transfer rate of Copper is higher than that of Aluminium.

Experimental Investigation of Heat transfer rate of Nano fluids using a Shell and Tube Heat exchanger

Nano fluids are used for increasing thermal properties in heat transfer equipment like heat exchangers, radiators etc. This paper investigates the heat transfer rate of Nano fluids using a shell and tube heat exchanger in single and multi tubes under turbulent flow condition by a forced convection mode. Alumina Nanoparticles are prepared by using Sol-Gel method. Heat transfer rate increases with decreasing particle size. In this experiment Alumina Nano particles of about 22 nm diameter used. Alumina Nano fluids are prepared with different concentrations of Alumina particles (0.13%, 0.27%, 0.4%, and 0.53%) with water as a base fluid using ultra-sonicator. Experiment have been conducted on shell and tube heat exchanger for the above concentrations on parallel and counter flow conditions by keeping constant inlet temperatures and mass flow rate. The result shows that the heat transfer rate is good compared to conventional fluids. The properties of Nano fluids and non-dimensional numbers have been calculated.

A Novel Prototype of a Small-Scale Solar Power Plant: dynamic Simulation and Thermoeconomic Analysis

The paper presents an innovative prototype of a solar power plant, purposely designed for small-scale applications such as residential and/or small commercial buildings. The system consists of 10 kWe Organic Rankine Cycle (ORC) plant and innovative solar thermal collectors. In particular, 235 m2 of flat-plate evacuated solar collectors are designed to heat diathermic oil up to 180°C. A variable volume pump is managed by a feedback controller in order to obtain the desired outlet set point temperature for the different weather/load conditions. The hot diathermic oil passes through a storage tank, which is adopted with the purpose to reduce the oscillations due to the solar radiation variability. By means of the tank, a better exploitation of solar energy is also achieved, reducing the time-shift between production and demand. For achieving a constant ORC inlet temperature the tank outlet hot diathermic oil passes through a gas-fired burner, which provides auxiliary additional thermal energy. Therefore, the ORC simultaneously produces electrical energy and low temperature (45°C) cogeneration heat. This solar power plant was dynamically simulated in TRNSYS environment. The simulation tool includes a novel model in order to simulate the dynamic thermal behavior of solar collectors, whereas the ORC dynamic was simulated by means of a performance data map provided by the manufacturer. The model also includes a tool for the calculation of energy and economic parameters and the evaluation of the optimum thermoeconomic configuration. Results indicated that the best pay-back period is obtained for a solar field area of about 200 m2 and a solar fraction of about 75%. Nevertheless, the system may be competitive from the economic point of view only if incentives can be made available, as commonly occurs for renewable applications, i.e., solar systems. This prototypal system aims to become a potential solar power system for small residential and/or commercial buildings. The presented study will be suitably used as a basis for the installation of a working prototype in located Naples, Italy.

Seasonal thermodynamic prediction of the performance of a hybrid solar gas-turbine power plant

An entirely thermodynamic model is developed for predicting the performance records of a solar hybrid gas turbine power plant with variable irradiance and ambient temperature conditions. The model considers a serial solar hybridization in those periods when solar irradiance is high enough. A combustion chamber allows to maintain an approximately constant inlet temperature in the turbine ensuring a stable power output. The overall plant thermal efficiency is written as a combination of the thermal efficiencies of the involved subsystems and the required heat exchangers. Numerical values of model input parameters are taken from a central tower installation recently developed near Seville, Spain. Real data for irradiance and external temperature are taken in hourly terms. The curves of several variables are obtained for representative days of all seasons: overall plant efficiency, solar subsystem efficiency, solar share, fuel conversion rate, and power output. The fuel consumption assuming natural gas fueling is calculated and the reduction in greenhouse emissions is discussed. The model can be applied to predict the daily and seasonal evolution of the performance of real installations in terms of a reduced set of parameters.

Experimental thermal behavior response of paraffin wax as storage unit

The paper presents experimental thermal behavior of paraffin wax as phase change material (PCM) using shell and tube with four-pass heat exchanger unit of 1 MJ storage unit under different flow rates. Thermal energy storage unit is constructed to study the performance of commercial grade paraffin white wax as energy storage material. The heat exchanger is constructed to obtain realistic energy storage behavior during charging and discharging cycle. The water is used as heat transfer fluid (HTF) to supply and absorb heat from PCM and to calculate energy stored. The charging and discharging experiments are carried out at constant inlet fluid temperature and constant pressure to observe the effects of flow rate of HTF on the performance of the storage unit at five different flow rates. The result shows that the charging time was in the range 195–310 min while discharging time in the range 80–38 min for change in flow rate from 35 to 75 LPH. The charging as well as discharging cycle time decreases with increase in mass flow rate of HTF, fall in cycle time during discharge is around three times faster than charging, and the same is attributed to fissure crack developed in the lump mass. Increase in flow rate decreases the heat storage.

Numerical investigation of the heat transfer and pressure drop on tube bundle support plates for inline and staggered arrangements

Heat transfer and pressure drop characteristics are investigated numerically on tube bundle support plates for inline and staggered arrangements. Three-dimensional numerical analyses within the framework of unsteady Reynolds averaged numerical simulations are performed using four different turbulence models, namely the realisable k-e, k-ω, SST and transition SST models. In order to reduce the computational effort, simplifications with respect to the domain definition are applied, which were validated within the framework of preliminary studies for assuring accuracy. A constant inlet temperature and a uniform heat flux on the support plates are applied as thermal boundary conditions. The Reynolds number is varied from 989 to 5,550, while keeping the Prandtl number constant at 0.70. The Nusselt number and the pressure drop values are obtained as functions of the Reynolds numbers. The effects of tube bundle arrangements on heat transfer and pressure drop are discussed. Results obtained using different turbulence models are compared with each other and with experiments. The best agreement with the experiments is obtained by the SST model. The least accurate results are provided by the k-e model. The remaining two turbulence models showed an acceptable performance, close to that of the SST model.

Energy, exergy and environmental analyses of a combined cycle power plant under part-load conditions

The aim of this study is to evaluate the performance of combined cycle power plants in full and part-load operations in terms of energy, exergy and environment. The environmental analysis is performed using life cycle assessment method. Two different strategies are selected to control the gas turbine under part-load conditions, including Turbine Exhaust Temperature (TET) constant and Turbine Inlet Temperature (TIT) constant modes. Results show that gas turbine has a higher efficiency in TET constant mode. But, combined cycle shows a better efficiency in TIT constant mode. Exergy analysis results show that combustion chamber has the highest rate of exergy destruction, both in full and part-load operation. With reducing the load, total exergy destruction of the plant is decreased which is due to lower fuel consumption. Also reducing the load by 50% leads to 2.47% and 6.36% reduction in exergy efficiency of the plant in TIT and TET constant modes, respectively. Therefore, in terms of energy and exergy, it is suggested to use the TIT constant mode for part-load operation of the gas turbine within a combined cycle. But in terms of environmental analysis, results show that more emissions are produced in TIT constant mode, due to higher fuel consumption.

Numerical simulation and analysis of confined turbulent buoyant jet with variable source

AbstractIn this work, experimental and numerical investigations are undertaken for confined buoyant turbulent jet with varying inlet temperatures. Results of the experimental work and numerical simulations for the problem under consideration are presented. Four cases of different variable inlet temperatures and different flow rates are considered. The realizable k-ɛ turbulence model is used to model the turbulent flow. Comparisons show good agreements between simulated and measured results. The average deviation of the simulated temperature by realizable k-ɛ turbulent model and the measured temperature is within 2%. The results indicate that temperatures along the vertical axis vary, generally, in nonlinear fashion as opposed to the approximately linear variation that was observed for the constant inlet temperature that was done in a previous work. Furthermore, thermal stratification exits, particularly closer to the entrance region. Further away from the entrance region the variation in temperatures becomes relatively smaller. The stratification is observed since the start of the experiment and continues during the whole course. Numerical experiments for constant, monotone increasing and monotone decreasing of inlet temperature are done to show its effect on the buoyancy force in terms of Richardson number.

Electrospun dual-layer nonwoven membrane for desalination by air gap membrane distillation

In the present study, dual-layer nanofiber nonwoven membranes were prepared by a facile electrospinning technique and applied for desalination by air gap membrane distillation (AGMD). Neat single and dual-layer nanofiber membranes composed of a hydrophobic polyvinylidene fluoride-co-hexafluoropropylene (PH) top layer with different supporting hydrophilic layer made of either polyvinyl alcohol (PVA), nylon-6 (N6), or polyacrylonitrile (PAN) nanofibers were fabricated with and without heat-press post-treatment. Surface characterization showed that the active layer (i.e., PH) of all electrospun nanofiber membranes (ENMs) exhibited a rough, highly porous (>80% porosity), and hydrophobic surface (CA>140°), while the other side was hydrophilic (CA<90°) with varying porosity. Heat-pressing the membrane resulted to thinner thickness (from >129μm to <100μm) and smaller pore sizes (<0.27μm). The AGMD experiments in a co-current flow set-up were carried out with constant inlet temperatures at the feed and permeate streams of 60±1.5 and 20±1.5°C, respectively. The AGMD module had a membrane area of 21cm2 and the thickness of the air gap was 3mm. The neat single and dual-layer ENMs showed a water permeate flux of about 10.9–15.5L/m2h (LMH) using 3.5wt.% NaCl solution as feed, which was much higher than that of a commercial PVDF membrane (~5LMH). The provision of a hydrophilic layer at the bottom layer enhanced the AGMD performance depending on the wettability and characteristics of the support layer. The PH/N6 dual-layer nanofiber membrane prepared under the optimum condition showed flux and salt rejection of 15.5LMH and 99.2%, respectively, which has good potential for AGMD application.

Hydrodynamics and Heat Transfer Characteristics of a Miniature Plate Pin-Fin Heat Sink Utilizing Al2O3–Water and TiO2–Water Nanofluids

In this paper, the hydrodynamic and thermal performance of a miniature plate pin-finned heat sink is investigated experimentally by utilizing two widely used nanofluids, Al2O3–water and TiO2–water. The heat sink base plate, which is used in the cooling process of electronic devices, has the dimensions of 42 mm (L) × 42 mm (W) × 14 mm (H) and is made of aluminum and placed in a plexiglass case which is isolated from the environment using an insulator foam. The thermal performance of the heat sink is investigated by passing the nanofluid at constant inlet temperature while applying a constant heat flux of 124.8 kW/m2 to the bottom surface of the heat sink. The nanofluids are prepared in volume concentrations of 0.5, 1, 1.5, and 2% and their performances are measured considering water as the base fluid. Measuring the pressure difference between the entrance and exit of the heat sink made it possible to study the hydrodynamic performance of the heat sink. Although the measurements showed 15% and 30% increase in the pumping power for the volume concentration of 2% of Al2O3–water and TiO2–water nanofluids, respectively, the average heat transfer coefficients increased by 16% and 14% and the thermal resistance decreased by 17% and 14% for each nanofluid.

Effect of Maltodextrin in Production of Spray Dried Kaffir Lime

This study investigated the effect amount of maltodextrin as a carrier agent in production of Kaffir Lime powder. Pure concentrated Citrus Hystrix juice with maltodextrin as carrier agent has been spray dried at constant inlet temperature of 140°C, outlet temperatute of 70°C and pressure of 40psi and pump speeds at 1 rpm with different percentage of maltodextrin. The highest amount of Kaffir Lime powder, 23.43 g was obtained at 40% of maltodextrin added to the juice.

Part load operation of a solid oxide electrolysis system for integration with renewable energy sources

100 % carbon-free energy economy might require the interconnection between electricity, gas and transport fuel energy systems and the development of large-scale energy storage systems. All of these can be achieved through electrolysis plants. These plants can produce hydrogen during renewable electricity surplus periods, which can be fed into the natural gas network or sold as transport fuel. In this study a complete model of a solid-oxide electrolysis process has been implemented in order to optimize the design of the process and its operation at part load. Different thermodynamic cell operational modes (constant cell inlet temperature vs. thermoneutral following point) and operational strategies (constant steam conversion vs. constant flow rate) have been analysed. Additionally, three different temperature-dependent area specific resistance equations have been implemented in order to analyse the influence of the dependency of the ionic conductivity of the electrolyte on temperature over the performance of the process.As result of the analysis, it has been seen that the system is able to operate from 10 to 100 % power load improving the compatibility of solid-oxide electrolysis systems with high transient renewable energy sources. The study reveals that the hydrogen production system can achieve a very flat performance curve within the whole power load range, with overall efficiencies between 91 % and 97 % vs. HHV. This is possible if at low power loads the electrolyser is operated under constant steam conversion mode; whereas at high loads, this is switched to constant flow rate mode. Finally, cells with high variation of area specific resistance with temperature suffers the lowest temperature variation along the power load. This makes possible to increase the operational power range and might reduce thermal stress increasing the lifespan of the cells.

Experimental and numerical investigation on a new type of heat exchanger in ground source heat pump system

Based on deficiencies of existing underground heat exchangers, a new design is proposed which consists of one outlet and three inlet pipes. Experimental measurements to evaluate the performance of single-U, double-U, and the new design referred to as 3I, for three inlet pipes, were carried out which showed superior performance of the 3I design. Experimental results showed that when under a constant heating load, the average circulating water temperature (ACWT) of the 3I-type was 3.7 and 1 °C lower than that of single-U and double-U, respectively. The thermal resistance of the 3I-type was 15.8 and 31.1 % lower than that of double-U and single-U type. In addition, numerical simulations were performed to further investigate the heat transfer performance over a very long operating time. Numerical results showed that for long-term operation under constant heating load, the ACWT of the 3I-type was 6.6 and 5.8 % lower than that of double-U type. Under a constant inlet water temperature that mimics a condenser output, the heat exchange per unit borehole depth of 3I-type was 17.1 and 11.6 % higher than that of double-U type in different operation conditions.

Short-term experiments with borehole heat exchangers and model validation in TRNSYS

The short-term behavior of borehole heat exchangers (BHE) is investigated using three 2 h test sequences each with constant inlet temperature and constant mass flow rate. Three models of the transient systems simulation program (TRNSYS) have been compared to these measurements. In order to assess the deviations a 3D-FEM model, built in COMSOL Multiphysics®, is compared to the measurement as well.The “Erdwärmesonden” model (EWS) shows a very good accuracy with a deviation less than 10%. The superposition borehole model (SBM) and duct ground heat storage model (DST) neglect the internal borehole heat capacity of fluid and grout. In order to improve their short-term behavior, the models are modified with an adiabatic pipe model with wall capacity in front of the BHE model. Different parameter sets for the pre-pipe are investigated. The optimum parameter set reduces the error between simulation and measurement of the injected heat from 50% to about 5%. With this model modification, all investigated models SBM, DST, and EWS perform as accurate as the COMSOL model.

Assessment of R-438A as a retrofit refrigerant for R-22 in direct expansion water chiller

The present work aims to evaluate the performance characteristics of a vapor compression refrigeration system using R-438A as a retrofit refrigerant for R-22. In order to achieve this objective, a test facility is developed and experiments are performed over a wide range of chilled water inlet temperature (11:20 °C), condenser water inlet temperature (25:35 °C) and condenser water mass flow rate (363:543 kg h−1). Results showed that as the chilled water inlet temperature changes from 11.5 to 20.5 °C, system COP increases from 1.78 to 2.07 at constant condenser water inlet temperature of 25.5 °C. Cooling capacity and COP of the system using R-438A are lower than R-22 by 11% and 12.5%, respectively. However, compressor discharge temperature using R-438A is slightly lower than R-22 which confirms that R-438A can be used as a retrofit refrigerant for R-22 to complete the remaining life time of the existing plants.

Abandoned petroleum wells as sustainable sources of geothermal energy

A reliable heat transfer model was developed to determine the performance of a double pipe heat exchanger retrofitted to an abandoned petroleum well. The proposed model is compared against an analytical heat transfer model and two numerical models, in order to determine the reliability and accuracy of the proposed model. Rock mass properties and well dimensions are based on realistic averages, with the geothermal gradient and depth of the well based on an abandoned well in the Persian Gulf. The proposed model makes use Fourier's diffusion law coupled with terms to account for the unsteady state of the model and the convective heat transfer. These three properties are coupled with the energy conservation equation and simulated with the finite element modeller FlexPDE. The proposed model was developed with a constant inlet temperature and a constant power configuration. The constant inlet temperature model is used to ascertain the effects of insulation, inlet fluid temperature, mass flow rate, thermal conductivity of the rock mass, geothermal gradient, and vertical groundwater flow. The constant power model is better suited to direct use and heat pump applications due to the requirement of a stable power source.

Mass flowrate characteristics of supercritical CO2 flowing through an electronic expansion valve

Electronic expansion valves (EEV) are employed increasingly in transcritical CO2 systems. In order to investigate the mass flowrate characteristics of supercritical CO2 flowing through an EEV, the effects of EEV outlet pressure on the mass flowrate of CO2 is studied under constant EEV inlet pressure and inlet temperature. The two-phase flow patterns of supercritical CO2 flowing through an EEV are revealed, and the conventional choked flow judgment formulas is proved effective for supercritical CO2. Based on Bernoulli Equation with expansion factor, the mass flowrate correlation of supercritical CO2 flowing through an EEV is presented. The influence of the expansion coefficient on the mass flowrate is considered in the correlation in which the correction coefficient is only affected by the EEV opening, the changing rate of the density and the pressure difference in the entrance region. The mass flowrate predicted by the correlation in this paper shows a good agreement with experimental data. The maximum relative errors are within 10%. The average and standard deviations are 0.76% and 5.9%, respectively.

The Effects of Design Parameters on Performance and NO Emissions of Steam-Injected Diesel Engine with Exhaust Gas Recirculation

In this study, an engine test with steam injection and EGR is conducted for investigating the effects of engine design parameters such as compression ratio (CR), charge temperature and pressure and equivalent ratio on a direct injection diesel engine performance and NO emissions using zero-dimensional single-zone combustion model. After the results obtained from the model are validated and well suited with those experimental studies of the diesel engines running with 20 % steam ratio and 10 % EGR, some important design parameters are investigated by using the theoretical model. As results, it was shown that effective efficiency, effective power and NO emissions increase with the increase of CR, inlet pressure for a constant equivalence ratio and inlet temperature. However, effective power and efficiency increase to certain values and then reduce, while NO emissions constantly decrease as the inlet pressure increases for various equivalence ratios.

Performance analysis of an ejector cooling system with a conventional chilled water system

The central chilled water system servicing one of the academic buildings at the KFUPM campus located in Dhahran has been modeled numerically to combine with the ejector cooling system. This ejector system cools the water which is in turn utilized to cool the refrigerant in the chiller condenser and hence, the function of the cooling tower in a conventional chilled water system is eliminated by an ejector cooling system. By replacing the cooling tower of the chilled water system with an ejector cooling system, constant water inlet temperature for the chiller condenser is assured, regardless of the ambient relative humidity and this effect is analyzed in detail. Results indicate that this combined system is quite advantageous at conditions of high ambient relative humidity when the cooling capacity of the cooling tower is greatly diminished but the performance of the ejector cooling system remains unaffected.

Collector Simulation Model with Dynamic Incidence Angle Modifier for Anisotropic Diffuse Irradiance

The incidence angle modifier (IAM) of a solar thermal collector for diffuse irradiance is usually determined under the simplifying assumption of isotropic sky and ground radiance. It is applied as one constant collector parameter, independent from slope or weather conditions. The simulation model introduced here considers the varying anisotropy of sky radiance. To create realistic distributions, the approach of Brunger and Hooper is used. Three modes are possible: Mode 1 calculates separate IAMs for anisotropic sky (for every time step) and isotropic ground. Mode 2 calculates separate IAMs for isotropic sky and isotropic ground (once per simulation). Mode 3 uses a user-specified isotropic IAM-value for the collector hemisphere.The model is applied to a stationary, double-covered process heat flat-plate collector with one-sided CPC booster reflector (RefleC). This collector shows a biaxial and asymmetric IAM for direct irradiance. It is found that, compared to anisotropic modeling, the simplified isotropic model is undervaluing the annual output of this collector by 13.7% for a constant inlet temperature of 120°C in Würzburg, Germany. At 40°C inlet temperature the undervaluation is 9.3%. For the basis flat-plate without reflector the undervaluation is 7.5% at 120°C and 3.3% at 40°C. An annual irradiation distribution diagram shows that this is due to an underestimation of diffuse irradiation from directions with high direct irradiation. Detailed results reveal that for RefleC the IAM for anisotropic diffuse sky radiance can vary by up to approx. 25 percentage points during one day.It is concluded that isotropic modeling of diffuse irradiance can be expected to significantly undervalue the annual output of all non-focusing solar thermal collectors. Highest relevance is found for high collector slopes, complex IAMs and at low-efficiency operation. The optimal collector slope is almost not affected. Accuracy of existing models can be increased by applying Mode 2.