Dynamic Modelling and Validation of an Air-to-Water Reversible R744 Heat Pump for High Energy Demand Buildings

This paper presents a reversible heat pump based on CO2 as the refrigerant, able to provide heating, cooling, and domestic hot water to high energy demand buildings. The unit was developed and tested under the EU H2020 project MultiPACK, which has the main goal of assuring the market about the feasibility, reliability, and energy efficiency of CO2 integrated systems for heating and cooling and promoting a fast transition to low environmental impact solutions. Within the project, the confidence raising was performed by installation and monitoring of fully integrated state-of-the art CO2 systems in the Southern European Climate. With the aim of predicting the unit behaviour under variable load and boundary conditions, a dynamic model of the entire unit was developed with commercial software, considering actual components and the implemented control system and it was validated with experimental data, collected at the factory’s lab before commissioning. The validation against experimental data collected during operation as a heat pump demonstrated a maximum percentage difference between the experimental and predicted value of gas–cooler heat flow rate equal to +5.0%. A preliminary comparison with the experimental data in chiller configuration is reported, however further development was required to achieve a satisfactory validation. Lastly, the numerical model was utilized to simulate a typical operation in heat pump configuration with the system coupled with a hot water tank storage for the production of domestic hot water and space heating; the model predicts higher COP when operating in domestic hot water operation due to the lower water inlet temperature.

The Effect of Plants on Extensive Green Roofs in Urban Heat Island Mitigation Efforts in Humid Tropical Cities

One of the urban heat island mitigation strategies in reducing urban temperatures in tropical cities is the application of a green roof system. This study compares the reduction in temperature and heat flow rate provided by three types of plants on extensive green roofs (EGR). We demonstrated that a EGR constructed with three types of plants (ground cover, and shrubs) could result in a decrease in temperature relative to the normal roof (NR). The results showed that the base temperature of the EGR of the bush and ground cover was lower than the base temperature of the NR which was 10.2ºC on indoor air, 17.8ºC on the inside and 19.1ºC on the outside. The peak indoor temperature was over 50ºC for the NR prototype. In the model with pennisetum purpureum schamach as the EGR, the maximum temperature was 40.1ºC, while for portulaca grandiflora and tradescantia spathacea the peaks were 37.6ºC and 37.5ºC, respectively. This shows that plants with large leaf widths are able to reduce heat greater than plants with small leaf widths.

Thermal Calculations of Four-Row Plate-Fin and Tube Heat Exchanger Taking into Account Different Air-Side Correlations on Individual Rows of Tubes for Low Reynold Numbers

Currently, when designing plate-fin and tube heat exchangers, only the average value of the heat transfer coefficient (HTC) is considered. However, each row of the heat exchanger (HEX) has different hydraulic–thermal characteristics. When the air velocity upstream of the HEX is lower than approximately 3 m/s, the exchanged heat flow rates at the first rows of tubes are higher than the average value for the entire HEX. The heat flow rate transferred in the first rows of tubes can reach up to 65% of the heat output of the entire exchanger. This article presents the method of determination of the individual correlations for the air-side Nusselt numbers on each row of tubes for a four-row finned HEX with continuous flat fins and round tubes in a staggered tube layout. The method was built based on CFD modelling using the numerical model of the designed HEX. Mass average temperatures for each row were simulated for over a dozen different airflow velocities from 0.3 m/s to 2.5 m/s. The correlations for the air-side Nusselt number on individual rows of tubes were determined using the least-squares method with a 95% confidence interval. The obtained correlations for the air-side Nusselt number on individual rows of tubes will enable the selection of the optimum number of tube rows for a given heat output of the HEX. The investment costs of the HEX can be reduced by decreasing the tube row number. Moreover, the operating costs of the HEX can also be lowered, as the air pressure losses on the HEX will be lower, which in turn enables the reduction in the air fan power.

Influence of variability in hygrothermal properties on analytical results of simultaneous heat and moisture transfer in porous materials

Depending on the data source used, the material hygrothermal properties that are used in the numerical analysis of simultaneous heat and moisture transfer will not be consistent. Differences in measurement methods and the individuality of specimens account for this. It is necessary to choose values from these different physical property sets to conduct a numerical calculation, which can cause the calculated results to differ. The subsequent range of variation in the calculated results should be quantitatively evaluated. In this study, the physical properties of several types of porous building materials were first gathered from four databases. The data were then categorized based on the kind of material and compared in terms of each physical property (density, porosity, specific heat, moisture capacity, thermal conductivity, and vapor permeability). The density, porosity, and specific heat varied by 10% on average, and the moisture capacity, thermal conductivity, and vapor permeability varied by 20% or more for all types of materials. In particular, the vapor permeability of plywood and moisture capacity of gypsum board differed by 50%. The influence that these physical property value variations had on hygrothermal calculation results was then quantitatively demonstrated for moisture and heat flow rate under a step change in the relative humidity or temperature of indoor air for a single layer wall. The moisture and heat flow rate into a single layer wall fluctuated by approximately 10%–40% due to differences in the vapor permeability and moisture capacity of the materials. For all types of materials, moisture was transferred more slowly than heat. Therefore, differences in moisture property values, such as vapor permeability and moisture capacity, influenced the results more significantly. Moreover, the moisture flow was accompanied by a phase change. The differences in moisture property values thus affected the heat flow.

CFD Analysis of Monolithic Heat Exchanger by Using Various Ceramic Materials

The main objective of this paper is to increase the heat exchange rate in the monolithic heat exchanger by changing the air passage shape and materials; here we used circular, hexagonal, oval shape for air passage and Al2O3, SiC CrCO3 ceramic materials for our research work. The heat flow rate is analyzed in CFD fluent software and model in done in CATIA software.

Wood and quartz substituted composite material characteristics

Many engineering designs focus on the production of durable, ergonomic, and economical new materials. In today’s world where natural resources are rapidly decreasing, recycling waste materials is of great importance. As a result of the bending test, the sample’s return to its original dimensions shows the plastic feature of the material. It was observed that the tensile strength can be increased if the speed of the injection machine is well adjusted in the production of tensile strength testing samples. There was an increase in the screw tensile strength of the samples and the joint hardness strength. Thermogravimetric analysis (TGA) showed that the reference samples were completely crumbled around 480 °C, and it was observed that only quartz remained from the input materials. Differential scanning calorimetry (DSC) showed that the peak point in the reference sample was at the heat flow rate of 29 m/W, while it was 18 to 19 m/W in the doped groups. Melting temperature was observed in the range of 125 to 135 °C in all groups. The mechanical properties of the quartz-substituted samples increased compared to the reference sample. It was observed that the quartz and wood powder contained in the mixture worked in harmony.

The mechanical aspects of bidirectional stretching on thermal performance in Burgers nanofluid flow subject to ohmic heating and chemical reaction

Thermal transport in 3D flow of Burgers nanofluid due to bidirectional stretching is an interesting topic with large number of applications. Motivated by this fact we formulated mathematical modelling for the 3D flow of viscoelastic Burgers nanofluid accelerated by bidirectional stretching surface. We studied the fluid relaxation and retardation time effects on the momentum and thermal transport of Burgers fluid. Moreover, we considered the effects of heat rise/fall and Ohmic heating to analyze the heat transport features in the flow of viscoelastic nanofluid. A momentous feature of this study is to incorporate the thermal relaxation time phenomenon to observe the properties of heat flow in nanofluid. Additionally, the mass transport phenomenon is explored by employing modified mass flux model and chemical reaction effects. Results are attained by employing homotopy analysis method (HAM) and illustrated through graphical representation. The main finding of the study exposes that the thermal transport in the flow is accelerated due to building strength of Eckert number [Formula: see text]. Moreover, the depreciating trend of concentration profiles is being detected for building strength of constructive chemical reaction parameter [Formula: see text]. Also, it is seen that the escalating magnitude of thermal relaxation time parameter [Formula: see text] serves to decline the heat flow rate.

Peloids as Thermotherapeutic Agents

The use of peloids as heat-providing therapeutic systems dates back to antiquity. Such systems consist of a liquid phase and an organic or inorganic solid phase. The latter facilitates the handling, preparation and stability of the solid–liquid system, modifying its organoleptic and phy-sicochemical properties, and improves its efficacy and tolerance. Peloids enable the application of heat to very specific zones and the release of heat at a given rate. The aims of this work are to study 16 reference peloids used in medical spa centers as thermo-therapeutic agents as well as to propose nine raw materials as a solid phase for the preparation of peloids. The physical properties studied are the centesimal composition, the instrumental texture and the thermal parameters. In conclusion, the peloids of the medical spas studied are used as thermotherapeutic agents in the treatment of musculoskeletal disorders, especially in knee osteoarthritis and to a lesser extent in back pain and psoriatic arthropathy. The clinical experience in these centers shows that the main effects of the application of their peloids are the reduction of pain, an increase in the joint’s functional capacity and an improvement in the quality of life. As thermotherapeutic agents, all the peloids of the me-dical spas studied and the pastes (raw materials with distilled water) examined showed a heat flow rate of up to four times lower than that shown by the same amount of water. The raw materials studied can be used as solid phases for the preparation of peloids with mineral waters.

Effect of heat generation and thermal radiation on heat transfer in porous enclosure having t-shape inner geometry

The numerical investigation of steady two-dimensional free convection is conducted to analyze the thermal radiation and viscous dissipation effects on heat transfer characteristics in fluid saturated T-shape porous hollow enclosure. The nonlinear partial differential equations in terms of stream function, using Darcy’s law and Boussinesq approximation, are solved numerically using finite difference scheme based on Gauss-Seidel approach. The results of this analysis discussed for the wide range of pertinent parameters such as radiation parameter ([Formula: see text]), viscous dissipation parameter ([Formula: see text]) and Rayleigh number ([Formula: see text]) in terms of local and average heat flow rate, streamlines and isotherms. The obtained results show that the average heat flow rate is enhanced with radiation parameter and reduced with viscous dissipation parameter. The results are graphically depicted to show the implications of the pertinent parameters in heat and flow field inside the hollow porous enclosure.

Circular Microchannel Heat Sink Optimization Using Entropy Generation Minimization Method

The performance of the microchannel heat sink (MCHS) in electronic applications needs to be optimized corresponding to the number of channels (N). In this study optimization of the number of channels corresponding to the diameter of the microchannel ({D_{N}}) using an entropy generation minimization approach is achieved for the MCHS used in electronic applications. The numerical study is performed for constant total heat flow rate ({\dot{q}_{tot}}) and total mass flow rate ({\dot{m}_{tot}}). The results indicate that the dominance of frictional entropy generation ({S_{gen,Fr}}) increases with the reduction in diameter. However, the entropy generation due to heat transfer ({S_{gen,HT}}) decreases with the reduction in diameter. Therefore, the optimum diameter ({D^{\ast }}) is calculated corresponding to the minimum total entropy generation ({S_{gen,total}}) for the optimum number of channels ({N^{\ast }}). Furthermore, the entropy generation number ({N_{S}}) and Bejan number (Be) are also calculated.