Simulation Study on the Heat Transfer Characteristics of a Spray-Cooled Single-Pipe Cooling Tower

The current study focuses on spray cooling applied to the heat exchange components of a cooling tower.An optimization of such processes is attempted by assessing different spray flow rates and droplet sizes.For simplicity,the heat exchanger of the cooling tower is modeled as a horizontal round tube and a cooling tower spray cooling model is developed accordingly using a computational fluid dynamics(CFD)software.The study examines the influence of varying spray flow rates and droplet sizes on the heat flow intensity between the liquid layer on the surface of the cylindrical tube and the surrounding air,taking into account the number of nozzles.It is observed that on increasing the spray flow strength,the heat flow intensity and extent of the liquid film in the system are enhanced accordingly.Moreover,the magnitude of droplet size significantly impacts heat transfer.A larger droplet size decreases evaporation in the air and enhances the deposition of droplets on the round tube.This facilitates the creation of the liquid film and enhances the passage of heat between the liquid film and air.Increasing the number of nozzles,while maintaining a constant spray flow rate,results in a decrease in the flow rate of each individual nozzle.This decrease is not favorable in terms of heat transfer.

Numerical study of the deep removal of R134a from non-condensable gas mixture by cryogenic condensation and de-sublimation

Nowadays,the limits on greenhouse gas emissions are becoming increasingly stringent.In present research,a two-dimensional numerical model was established to simulate the deep removal of 1,1,1,2-tetrafluoroethane(R134a)from the non-condensable gas(NCG)mixture by cryogenic condensation and de-sublimation.The wall condensation method was compiled into the Fluent software to calculate the condensation of R134a from the gas mixture.Besides,the saturated thermodynamic properties of R134a under its triple point were extrapolated by the equation of state.The simulation of the steam condensation with NCG was conducted to verify the validity of the model,the results matched well with the experimental data.Subsequently,the condensation characteristics of R134a with NCG and the thermodynamic parameters affecting condensation were studied.The results show that the section with relatively higher removal efficiency is usually near the inlet.The cold wall temperature has a great influence on the R134a removal performance,e.g.,a 15 K reduction of the wall temperature brings a reduction in the outlet R134a molar fraction by 85.43%.The effect of changing mass flow rate on R134a removal is mainly reflected at the outlet,where an increase in mass flow rate of 12.6% can aggravate the outlet molar fraction to 210.3% of the original.The research can provide a valuable reference for the simulation of the deep removal of various low-concentration gas using condensation and de-sublimation methods.

Physical Properties in Drying of Food Products with Combined Sublimation and Evaporation

Drying is an important unit operation in processing of biological resources. The drying process may influence the product properties and quality, which may shrink, break or undergo rheological, physical and biochemical changes. The important parameters responsible for such changes are drying conditions, type of drying technology and residence drying time. Thermal conductivity, thermal-mass diffusivity, enthalpy, porosity and density are the main material property and heat-mass transfer parameters, which are essential for understanding the changes in product quality and for designing and dimensioning the drying processes. In this paper physical properties of food products undergoing a combined sublimation and evaporation were studied. Pieces of vegetables and potatoes were dried in a heat pump fluidized bed dryer at combined modes with temperatures below the freezing point in the beginning and a final drying step at temperatures above the freezing point. Samples of products were tested at different moisture contents with respect to physical properties. Physical properties of leek and potato samples were measured and mass diffusivities were determined from drying kinetic data. Based on bulk density and rehydration measurements it was clearly observed that drying temperature and modes influenced the final product physical properties. The potato cube run dried with initial atmospheric freeze-drying step had rehydration ability 430%above a run dried only above the freezing point. The average effective mass diffusivity for 5 mm slabs of leek was 0.5 x 10-11m2·s-1 for the sublimation stage and 2.2 x 10-11m2·s-1 for the evaporation stage.

PIV Investigations of Flow Patterns in the Entrance Configuration of Plate-fin Heat Exchanger

Flow characteristics in the entrance of plate-fin heat exchanger have been investigated by means of particle image velocimetry (PIV). The flow field was measured using the two-frame cross-correlation technique. Streamline and velocity contour graphs at different cross-sections were obtained in the experiment. The experimental results indicate that flow maldistribution in the conventional header is very serious, while the improved header configuration with punched baffle can effectively improve the uniformity. The flow maldistribution parameter in plate-fin heat exchanger has been reduced from 1.21 to 0.21, and the ratio of the maximum velocity to the minimum is reduced from 23.2 to 1.8 by install-ing the punched baffle. The results suggest room for the optimum design of plate-fin heat exchanger.

Performance Investigation of Capillary Tubes for Machine Tool Coolers Retrofitted with HFC-407C Refrigerant

The machine tool coolers are the best managers of coolant temperature in avoiding the deviation of spindle centerline for machine tools. However, the machine coolers are facing the compressed schedule to phase out the HCFC （hydro-chloro-floro-carbon） refrigerant and little attention has been paid to comparative study on sizing capillary tube for retrofitted HFC （hydro-floro-carbon） refrigerant. In this paper, the adiabatic flow in capillary tube is analyzed and modeled for retrofitting of HFC-407C refrigerant in a machine tool cooler system. A computer code including determining the length of sub-cooled flow region and the two phase region of capillary tube is developed. Comparative study of HCFC-22 and HFC-407C in a capillary tube is derived and conducted to simplify the traditional trial-and-error method of predicting the length of capillary tubes. Besides, experimental investigation is carried out by field tests to verify the simulation model and cooling performance of the machine tool cooler system. The results from the experiments reveal that the numerical model provides an effective approach to determine the performance data of capillary tube specific for retrofitting a HFC-407C machine tool cooler. The developed machine tool cooler system is not only directly compatible with new HFC-407C refrigerant, but can also perform a cost-effective temperature control specific for industrial machines.

Experimental Investigation of Capacity Control Scheme on System Performance for a Machine Tool Cooler

Highly accurate manufacture in machining industry can only be obtained with precise temperature control of the coolant （oil or water）.Machine tool with more accurate,stable and advanced the precision of the working component cannot be developed without appropriate cooling.However,the machine tool coolers are facing the control hunting of cooling temperature and the dramatic variation of heat load in high-accuracy machining.The main objective of this study is to evaluate the influence of the hot-gas by-pass scheme and suction regulation for capacity control of a machine tool cooler system.In this study,experimental investigation on both hot-gas by-pass scheme and suction valve regulation for capacity control has been proposed.Effects of using capillary tube and thermostatic expansion valve along with different capacity control scheme have been investigated extensively in an environmental testing room.Cooling performance and power consumption of the cooler system have been measured and analyzed as well by comparing with different opening percentage of throttling valve under specific coolant temperature.The experimental results reveal that the power consumption will reduce slightly by capacity control using the hot-gas by-pass scheme but the coefficient of performance （COP） of the overall system will decrease.Lower coolant temperature will result in higher compressor power consumption as well.While conducting suction valve regulating for capacity control,energy-saving at 10%-12% can be obtained by using thermostatic expansion valve under different evaporator load.It also reveals that suction valve regulation along with adequate choice of thermostatic expansion valve can provide alternative choice for steady capacity control and substantial energy-saving.The proposed cooler systems with different capacity control schemes are not only more cost-effective than inverter driven system,but also can perform energy-saving and precise temperature control specific for high-accuracy machine tool cooling.

A new empirical equation of shear wave velocity to predict the different peak surface accelerations for Jakarta city

Site condition and bedrock depth play important roles in the determination of peak surface acceleration(PSA)values by earthquake motions.The soil parameters of shear wave velocity(Vs)and standard penetration test-number(N)value for Jakarta city are available up to 100 m below the Earth’s surface even though the typical depths to bedrock are in excess of 100 m.This study referred to the base motion peak ground acceleration(PGA)values of 0.100 g,0.218 g and 0.378 g to predict the PSA values using the Nonlinear Earthquake site Response Analysis(NERA)to analyse a simulated dataset for the bedrock depths of 100 m,200 m,300 m,400 m and 500 m with conditioned by clayey and sandy soils.A new empirical equation of Vs=102.48 N0.297(m/s)was proposed to calculate the values of Vsused as an input parameter in the NERA programme for the prediction of seismic wave propagation.The results showed that the PSA values are dependent on the amplitude of seismic waves,depths of bedrock and the local site conditions.Changes in the PSA values from 41.0%to 51.5%and from 46.1%to 79.8%for the bedrocks overlain by sand,from 20.0%to 42.1%and from 45.9%to 58.8%for the bedrocks overlain by clay with increasing of bedrock depths from 200 m to 300 m and from 400 m to 500 m,respectively,were predicted for a 2500-year return period earthquake.Decreases in the PSA values by 41.0%,51.5%,46.1%,79.8%for the bedrocks overlain by sand and by 20.0%,42.1%,45.9%,58.8%for the bedrocks overlain by clay were predicted for a 2500-year return period earthquake due to the bedrock depth changes of 200 m,300 m,400 m,500 m.Large-magnitude earthquake of Jakarta city has a significant effect on an increase or a decrease of the PSA value with depth of bedrock and may cause the vibration damage to buildings and other constructions on the ground.The analysis of the PSA value and PSA ratio influenced by the PGA value,bedrock depth and local soil conditions will make a contribution to the design of earthquake-safe building for Jakarta city in the future.

Energy-Efficient Approaches for a Machine Tool Building in a University through Field Measurement and Energy Modelling

The heating,ventilating,and air conditioning(HVAC)system consumes nearly 50%of the building’s energy,especially in Taiwan with a hot and humid climate.Due to the challenges in obtaining energy sources and the negative impacts of excessive energy use on the environment,it is essential to employ an energy-efficient HVAC system.This study conducted the machine tools building in a university.The field measurement was carried out,and the data were used to conduct energymodelling with EnergyPlus(EP)in order to discover some improvements in energy-efficient design.The validation between fieldmeasurement and energymodelling was performed,and the error rate was less than 10%.The following strategies were proposed in this study based on several energy-efficient approaches,including room temperature settings,chilled water supply temperature settings,chiller coefficient of performance(COP),shading,and building location.Energy-efficient approaches have been evaluated and could reduce energy consumption annually.The results reveal that the proposed energy-efficient approaches of room temperature settings(3.8%),chilled water supply temperature settings(2.1%),chiller COP(5.9%),using shading(9.1%),and building location(3.0%),respectively,could reduce energy consumption.The analysis discovered that using a well-performing HVAC system and building shading were effective in lowering the amount of energy used,and the energy modelling method could be an effective and satisfactory tool in determining potential energy savings.

Thermodynamic performance of a cryogenic energy storage system based on natural gas liquefaction

Cryogenic energy storage(CES)is a viable method for grid-scale electrical energy storage.Considering the high energy density and mature application of liquefied natural gas(LNG),we proposed an LNG cryogenic energy storage(LNGES)system.A steady-state process model of the LNGES system was established using Aspen HYSYS.The effects of the natural gas composition and key operating parameters such as the charging pressure,discharging pressure,throttling temperature,and liquid storage pressure on the system performance were investigated.A multi-parameter genetic algorithm model built using the MATLAB software was used to optimize the LNGES system to optimize the round-trip efficiency(RTE).Then,an exergy analysis of the optimal configuration was conducted.The results suggested that the LNGES system could achieve optimal RTE and exergy efficiency values of 60.14%and 71.64%,respectively.Exergy destruction mainly occurred during the compression,throttling,expansion,and heat exchange.The proposed LNGES system could be a promising candidate for the large-scale application of CES technology in power grids and gas networks.

Thermodynamic Analysis of a Modified Ejector-Expansion Refrigeration Cycle with Hot Vapor Bypass

In this study,a modified ejector-expansion refrigeration cycle(MERC)is proposed for applications in small refrigeration units.A vapor bypass circuit is introduced into the standard ejector expansion refrigeration cycle(ERC)for increasing the ejector pressure lift ratio,thereby lowering the compressor pressure ratio in the MERC.A mathematical model has been established to evaluate the performances of MERC.Analysis results indicate that since a two phase vapor-liquid stream is used to drive the ejector in the MERC,a larger ejector pressure lift ratio can be achieved.Thus,the compressor pressure ratio decreases by 21.1%and the discharge temperature reduces from 93.6℃to 82.1℃ at the evaporating temperature of-55℃ when the vapor quality of two phase vapor-liquid stream increases from 0 to 0.2.In addition,the results show that the higher ejector component efficiencies are effective to reduce the compressor pressure ratio and the discharge temperature.Actually,the discharge temperature reduces from 91.4℃ to 82.1℃ with the ejector component efficiencies increasing from 0.75 to 0.85 at the two phase stream vapor quality of 0.2.Overall,the proposed cycle is found to be feasible in lower evaporating temperature cases.

An active pipe-embedded building envelope for utilizing low-grade energy sources

An active pipe-embedded building envelope, which is an external wall or roof with pipes embedded inside, was presented. This structure may utilize the circulating water in the pipe to transfer heat or coolth inside directly. This kind of structure is named ＂active pipe-embedded building envelope＂ due to dealing with the thermal energy actively inside the structure mass by circulating water. This structure not only deals with thermal energy before the external disturbance becomes cooling/heating load by using the circulating water, but also may use low-grade energy sources such as evaporative cooling, solar energy, and geothermal energy. In the meantime, this structure can also improve the indoor thermal comfort by tempering the internal wall surface temperature variation due to the thermal removal in the mass. This work further presents the thermal performance of this structure under a typical hot summer weather condition by comparing it with that of the conventional external wall/roof with numerical simulation. The results show that this pipe-embedded structure may reduce the external heat transfer significantly and reduce the internal wall surface temperature for improving thermal comfort. This work also presents the effects of the water temperature and the pipe spacing on the heat transfer of this structure. The internal surface heat transfer may reduce by about 2.6 W/mE when the water temperature reduces by 1℃ as far as a brick wall with pipes embedded inside is concerned. When the pipe spacing reduces by 50 mm, the internal wall surface heat flux can also reduce by about 2.3 W/m2.

Performance Analysis of an Ejector Enhanced Two-Stage Auto-Cascade Refrigeration Cycle for Low Temperature Freezer

In this paper,an ejector enhanced two-stage auto-cascade refrigeration cycle(EARC)using ternary mixture R600a/R32/R1150 is proposed for application of-80℃freezing.In EARC cycle,an ejector was employed to recover the expansion work in the throttling processes and lifted the suction pressure of the compressor.The performances of the ejector enhanced two-stage auto-cascade refrigeration cycle and conventional auto-cascade refrigeration cycle(CARC)were compared using thermodynamic analysis methods.The influences of the important operation parameters including mass fraction ratio of the mixture,fluid quality at the second separator inlet,condensation temperature,evaporation temperature,and expansion ratio of expansion valve on the performances of EARC cycle were discussed in detail.The results indicate that ternary mixture R600a/R32/R1150 has the optimal mass fraction ratio of 0.45/0.2/0.35 with respect to the maximum COP.The EARC cycle yields higher performance than the CARC cycle in terms of COP,exergy efficiency and volumetric refrigeration capacity.And 4.9%-36.5%improvement in COP and 6.9%-34.3%higher exergy efficiency could be obtained in EARC cycle comparing with CARC cycle.The finding of this study suggests that the EARC cycle has a promising application potential for low temperature freezing.

Fuzzy logic controller design with unevenly-distributed membership function for high performance chamber cooling system

Fuzzy logic controller adopting unevenly-distributed membership function was presented with the purpose of enhancing performance of the temperature control precision and robustness for the chamber cooling system.Histogram equalization and noise detection were performed to modify the evenly-distributed membership functions of error and error change rate into unevenly-distributed membership functions.Then,the experimental results with evenly and unevenly distributed membership functions were compared under the same outside environment conditions.The experimental results show that the steady-state error is reduced around 40% and the noise disturbance is rejected successfully even though noise range is 60% of the control precision range.The control precision is improved by reducing the steady-state error and the robustness is enhanced by rejecting noise disturbance through the fuzzy logic controller with unevenly-distributed membership function.Moreover,the system energy efficiency and lifetime of electronic expansion valve(EEV) installed in chamber cooling system are improved by adopting the unevenly-distributed membership function.

Simulation and analysis of a peak regulation gas power plant with advanced energy storage and cryogenic CO_(2) capture

Flexible gas power plants are subject to energy storage,peak regulations,and greenhouse gas emissions.This study proposes an integrated power generation system that combines liquid air energy storage(LAES),liquefied natural gas(LNG)cold energy utilization,gas power systems,and CO_(2) capture and storage(CCS)technologies,named the LAES-LNG-CCS system.The off-peak electricity can be stored in liquid air.During the peak period,air and gas turbines generate supplementary electricity.Both LNG chemical energy and cold energy were considered:the former was used for gas power plants,and the latter was used for LAES regasification and CCS processes.Based on the thermodynamic analysis,we evaluated the effects of the recovery pressure,CCS pressure,and combustion temperature on the system power consumption and efficiency.The results demonstrated that the system recovery pressure,CCS pressure,and combustion temperature had the greatest effects on system power generation.Round-trip efficiency(RTE)was significantly affected by combustion temperature.The largest exergy loss occurred in the gas power plant.The optimal system operating ranges of the system recovery pressure,CCS pressure,and combustion temperature were 6−10 MPa,0.53−0.8 MPa,and 1,503−1,773 K,where the RTEs and𝜂Ex,RS reached 55%−58.98%and 74.6%−76%,respectively.The proposed system can simultaneously achieve the synergistic functions of large-scale energy storage,multilevel energy utilization,peak regulation,and carbon emission reduction.It can also be widely used in advanced distributed energy storage applications in the future.

Comparative study on the globally optimal performance of cryogenic energy storage systems with different working media

Cryogenic energy storage(CES)has garnered attention as a large-scale electric energy storage technology for the storage and regulation of intermittent renewable electric energy in power networks.Nitrogen and argon can be found in the air,whereas methane is the primary component of natural gas,an important clean energy resource.Most research on CES focuses on liquid air energy storage(LAES),with its typical round-trip efficiency(RTE)being approximately 50%(theoretical).This study aims to explore the feasibility of using different gases as working media in CES systems,and consequently,to achieve a high system efficiency by constructing four steady-state process models for the CES systems with air,nitrogen,argon,and methane as working media using Aspen HYSYS.A combined single-parameter analysis and multi-parameter global optimization method was used for system optimization.Further,a group of key independent variables were analysed carefully to determine their reasonable ranges to achieve the ideal system performance,that is,RTE and liquefaction ratio through a single-parameter analysis.Consequently,a multi-parameter genetic algorithm was adopted to globally optimize the CES systems with different working media,and the energy and exergy analyses were conducted for the CES systems under their optimal conditions.The results indicated the high cycle efficiency of methane and a low irreversible loss in the liquefaction cycle.Moreover,the Joule-Thomson valve inlet temperature and charging and discharging pressures considerably affected the system performance.However,exergy loss in the CES system occurred primarily in the compressor,turbine,and liquefaction processes.The maximum optimal RTE of 55.84%was achieved in the liquid methane energy storage(LMES)system.Therefore,the LMES system is expected to exhibit potential for application in the CES technology to realize the integration of natural gas pipelines with renewable power grids on a large scale.Moreover,the results of study have important theoretical significance for the innovation of the CES technology.

Theoretical Performance Analysis of an Ejector Enhanced High-Temperature Heat Pump with Dual-Pressure Condensation and Evaporation

In this paper,an ejector enhanced high-temperature heat pump with dual-pressure condensation and evaporation is proposed to improve the system performance.Theoretical analyses of the system operation characteristics are conducted using energetic and exergetic methods.The performance comparisons among the basic cycle,parallel compression cycle,and ejector enhanced cycle are conducted with six different refrigerants,including R245fa,R600a,R1234ze(Z),R1336mzz(Z),R1224yd(Z),and R1233zd(E).The results demonstrate that environmentally-friendly refrigerant R1234ze(Z)would be a promising alternative refrigerant.Compared with the basic cycle and parallel compression cycle at selected operation conditions,29.5%and 12.6%improvements in COP,and 16.7%and 11.1%higher system exergy efficiency are achieved in the ejector enhanced cycle on average.The volumetric heating capacity of the ejector enhanced cycle is increased by 15.7%–21.7%.The ejector enhanced cycle outperforms the other two cycles in high-temperature heat pump applications at the large temperature lift and temperature rise in the heat sink.The assessment offers an option to improve the energy utilization efficiency of the high-temperature heat pumps.

Microfluidic mixing enhancement using electrokinetic instability under electric field perturbations in a double T-shaped microchannel

The electrokinetic instability(EKI) phenomenon occurs when microfluidic flows with an electrical conductivity gradient are driven by a high-intensity external electrical field. Although EKI limits the robust performance of complex electrokinetic bioanalytical systems,it can be actively exploited to achieve the rapid mixing of micro-and nanoliter volume solutions in microscale devices. This paper investigates the EKI phenomenon in a double T-shaped microchannel,in which two aqueous electrolyte solutions with a 3.5:1 conductivity ratio are driven electrokinetically into the mixing channel via the application of a DC electrical field. A stratified flow condition is formed when the intensity of the applied DC electrical field is below a certain threshold value. However,as the intensity is increased,a series of flow circulations forms at the interfaces of neighboring solutions flows,and then propagates in the downstream direction when the intensity of the electrical field is increased beyond a certain critical threshold value. Electrical field intensity perturbations aligned in the direction of the conductivity gradient are then added to the DC electrical field at the upper inlet of the double T-shaped microchannel near the main mixing channel. It is found that these perturbations can stir the microfluidic instability and the induced flow instability conditions can enhance the mixing efficiency.

A fully solid-state cold thermal energy storage device for car seats using shape-memory alloys

Thermal energy storage has been a pivotal technology to fill the gap between energy demands and energy supplies.As a solid-solid phase change material,shape-memory alloys(SMAs)have the inherent advantages of leakage free,no encapsulation,negligible volume variation,as well as superior energy storage properties such as high thermal conductivity(compared with ice and paraffin)and volumetric energy density,making them excellent thermal energy storage materials.Considering these characteristics,the design of the shape-memory alloy based the cold thermal energy storage system for precooling car seat application is introduced in this paper based on the proposed shape-memory alloy-based cold thermal energy storage cycle.The simulation results show that the minimum temperature of the metal boss under the seat reaches 26.2°C at 9.85 s,which is reduced by 9.8°C,and the energy storage efficiency of the device is 66%.The influence of initial temperature,elastocaloric materials,and the shape-memory alloy geometry scheme on the performance of car seat cold thermal energy storage devices is also discussed.Since SMAs are both solid-state refrigerants and thermal energy storage materials,hopefully the proposed concept can promote the development of more promising shape-memory alloy-based cold and hot thermal energy storage devices.

A compact elastocaloric refrigerator

Elastocaloric cooling is regarded as one of the most promising cutting-edge alternatives to conventional vapor compression refrigeration systems.This technology is based on the temperature change of materials when being subjected to uniaxial stress,which has been observed in polymers,alloys,and ceramics.However,the existing elastocaloric prototypes have a bottleneck problem of an excessive mass ratio between the actuator and the solid-state refrigerant.

Combining magnetocaloric and elastocaloric effects in a Ni45Co5Mn37In13 alloy

1.Introduction Refrigeration plays an essential role in nowadays society.However,conventional refrigeration based on vapor compression cooling shows high energy consumption,complicated structure and even environmental pollution.Searching for an environmentalfriendly and energy-saving refrigeration technology has become a matter of concern[1,2].In the past few decades,the solid-state refrigeration technologies mainly represented by magnetocaloric effect(MCE),electrocaloric effect(ECE)and elastocaloric effect(e CE)have been regarded as promising candidate to replace traditional vapor compression refrigeration technology owing to environmental friendliness and high efficiency[1-9].