Application of neural networks to predict the transient performance of a Run-Around Membrane Energy Exchanger for yearly non-stop operation

Application of soft computing methods (i.e. neural networks and genetic algorithms) for modeling and controlling the dynamic and transient behavior of systems has been increasing during the last decade. In this study, a neural network (NN) model is developed to predict the transient heat and moisture transfer performances (i.e., the sensible and latent effectivenesses) of a novel HVAC energy exchanger, called the Run-Around Membrane Energy Exchanger (RAMEE), which is able to transfer both heat and moisture between exhaust and supply air streams. The training data set for the NN model covers a wide range of outdoor conditions and system parameters and is produced using a Transient Numerical Model (TNM) that has been experimentally validated for some transient applications. Two separate NNs (one for sensible and one for latent energy transfer) each with 12 inputs and one output, are selected to represent the RAMEE. The ability of NN models to predict the performance of a given RAMEE design in different climates is numerically validated. The mean absolute difference (MAD) between the results of TNM and NN models for different locations are 0.5 °C for the sensible model and 0.2 g_v/kg_a for the latent model, which indicates satisfactory agreement for energy exchange calculations. These NN models are very fast and easy to use therefore, they might be used for design purposes or estimating the annual energy savings in different buildings with continuous operation and a RAMEE in their HVAC system.     

Modeling of heat transfer and energy analysis of potato slices and cylinders during solar drying

In the present work, a method based on energy balance considering the effects of heat capacity of the food product, radiative heat transfer from food product to the drying chamber and solar radiation absorbed in the product during drying is proposed for determination of convective heat transfer coefficient, h_c. A natural convection mixed-mode solar dryer is used for performing the experiments on potato cylinders and slices of same thickness of 0.01 m with respective length and diameter of 0.05 m. The present investigation indicates that the cylindrical samples exhibit higher values of h_c and faster drying rate compared to those of slices, as expected. The h_c values for each sample shape are correlated by an equation of the form Nu = C(Ra)ⁿ. Laplace transform is applied to solve the proposed heat transfer diffusion model considering the effect of moisture transfer rate to predict the transient sample temperature. The model is validated through a close agreement between calculated and experimental results of transient sample temperature. Results of energy analysis reveal that for both the sample geometries, decreasing product moisture content during drying resulted in significant reduction in specific energy consumption. For almost similar drying conditions, a considerable amount of reduction in specific energy consumption is achieved for cylinders, as expected.     

Flow over and forced convection heat transfer in Newtonian fluids from a semi-circular cylinder

Momentum and heat transfer characteristics of a semi-circular cylinder immersed in unconfined flowing Newtonian fluids have been investigated numerically. The governing equations, namely, continuity, Navier–Stokes and energy, have been solved in the steady flow regime over wide ranges of the Reynolds number (0.01 ? Re ? 39.5) and Prandtl number (Pr ? 100). Prior to the investigation of drag and heat transfer phenomena, the critical values of the Reynolds number for wake formation (0.55 < Re^c < 0.6) and for the onset of vortex shedding (39.5 < Re_c < 40) have been identified. The corresponding values of the lift coefficient, drag coefficient, and Strouhal number are also presented. After establishing the limit of the steady flow regime, the influence of the Reynolds number (0.01 ? Re ? 39.5) and Prandtl number (Pr = 0.72, 1, 10, 50 and 100) on the global flow and heat transfer characteristics have been elucidated. Detailed kinematics of the flow is investigated in terms of the streamline and vorticity profiles and the variation of pressure coefficient in the vicinity of the cylinder. The functional dependence of the individual and total drag coefficients on the Reynolds number is explored. The Nusselt number shows an additional dependence on the Prandtl number. In addition, the isotherm profiles, local Nusselt number (Nu_L) and average Nusselt number (Nu) are also presented to analyze the heat transfer characteristic of a semi-circular cylinder in Newtonian media.     

The particle thermal conductivity influence of nanofluids on thermal performance of the microtubes

The paper presents the numerical analysis on microchannel laminar heat transfer and fluid flow of nanofluids in order to evaluate the suitable thermal conductivity of the nanoparticles that results in superior thermal performances compared to the base fluid. The diameter ratio of the micro-tube was D_i/D_o = 0.3/0.5 mm with a tube length L = 100 mm in order to avoid the heat dissipation effect. The heat transfer rate was fixed to Q = 2 W. The water based Al₂O₃, TiO₂ and Cu nanofluids were considered with various volume concentrations ϕ = 1,3 and 5% and two diameters of the particles d_p = 13 nm and 36 nm. The analysis is based on a fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results reveal that only the nanofluids with particles having very high thermal conductivity (λ_Cu = 401 W/m K) are justified for using in microcooling systems. Moreover, the analysis is sensitive to both the comparison criteria (Re or Π) and heat transfer parameters (h_ave or t_max).     

Spray water cooling heat transfer at high temperatures and liquid mass fluxes

Spray water cooling is an important technology used in industry for the cooling of materials from temperatures up to 1800 K. The heat transfer coefficient in the so-called steady film boiling regime is known to be a function of the water impact density. Below a specific surface temperature T_L, the heat transfer coefficient shows a strong dependence on temperature (Leidenfrost effect). These findings are the results of complex self-organizing two-phase boiling heat transfer phenomena.The heat transfer coefficient was measured by an automated cooling test stand (instationary method) under clean (non-oxidizing) surface conditions. Compared to the common thought, an additional temperature dependency in the high temperature regime was found. The heat transfer from the material to the outflowing spray water is explained by a simple model of the two-phase flow region. From the experimental data, an analytic correlation for the dependence of the heat transfer coefficient α as an analytic function of water impact density V_S and temperature ΔT is provided.For water temperatures around 291 K, surface temperatures between 473 and 1373 K, i.e. ΔT > 180 K and water impact densities between V_S = 3 and 30 kg/(m² s) the heat transfer coefficient α was measured. The spray was produced with full cone nozzles (v_d ≈ 13–15 m/s, d_d ≈ 300–400 μm).     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

Models of fluidized bed drying for thin-layer chopped coconut

Methods for efficiently drying agricultural products are in ever-increasing demand. Due to its thorough mixing ability, a fluidized bed technique was employed to evaluate the drying kinetics of thin-layer chopped coconut. The experiments were conducted at drying temperatures of 60–120 °C and a constant velocity of 2.5 m/s. Chopped coconut was dried from about 105% d.b. to approximately 3% d.b. The moisture transport phenomenon in fluidized bed thin-layer drying is described by immense acceleration in MR diminution in the early stage of drying, followed by considerable deceleration. Falling-rate drying, an outgrowth of restraining moisture transfer via internal mass-diffusion mechanism, thoroughly characterized chopped coconut drying. Among the 10 selected models, statistic analysis inferred that the Modified Henderson and Pabis model could predict changes in moisture content most accurately. Compared with the values of D_eff derived from Fick’s law for other food and biological materials usually dried in conventional tray dryers, the current values (5.9902 × 10^?8–2.6616 × 10^?7 m²/s) were substantially high, principally attributable to the unique characteristic of fluidized bed drying, remarkably encouraging heat and mass transfer. Activation energy was also described.     

Investigation of coated tubes in cross-flow boiling

The boiling in cross-flow is investigated for coated tubes (low-porosity, flame-sprayed) in this paper. The effect of surface roughness on flow boiling heat transfer for a horizontal tube surface in cross-flow is studied for saturated boiling of water at atmospheric pressure. The parameters varied were for flow velocity up to 3.24 kg/s (G = 258.49 kg/m² s), heat flux from 12 to 45 kW/m², surface roughness (Ra) from 0.3296 to 4.731 μm. Nominal enhancement in heat transfer coefficient at higher mass flux may be attributed to the continued nucleation at the uppermost surfaces (in the wake region of the flow) of the rougher tubes thereby increasing the overall heat transfer rate. The flow boiling data was found to best fit the Kutateladze asymptotic equation h = h_l[1 + (h_npb/h_l)ⁿ]^1/n with the value of n = 2.258 (which is close to the value of n = 2 suggested by Kutateladze).     

A harmonized calculation model for transforming EU bottom-up energy efficiency indicators into empirical estimates of policy impacts

This study is an impact analysis of European Union (EU) energy efficiency policy that employs both top-down energy consumption data and bottom-up energy efficiency statistics or indicators. As such, it may be considered a contribution to the effort called for in the EU's 2006 Energy Services Directive (ESD) to develop a harmonized calculation model. Although this study does not estimate the realized savings from individual policy measures, it does provide estimates of realized energy savings for energy efficiency policy measures in aggregate. Using fixed effects panel models, the annual cumulative savings in 2011 of combined household and manufacturing sector electricity and natural gas usage attributed to EU energy efficiency policies since 2000 is estimated to be 1136 PJ; the savings attributed to energy efficiency policies since 2006 is estimated to be 807 PJ, or the equivalent of 5.6% of 2011 EU energy consumption. As well as its contribution to energy efficiency policy analysis, this study adds to the development of methods that can improve the quality of information provided by standardized energy efficiency and sustainable resource indexes.     

Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel

Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.15 ? w_in ? 0.16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 × 10⁷ < Ra¹ < 8 × 10⁸ and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H_f/H = 0.25 to H_f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8–9 mm and optimum fin spacing depends on the value of Ra¹.     

The status of energy conservation in Taiwan's cement industry

The cement industry represents one of the most energy intensive sectors in Taiwan. Energy audits are the direct tools which are employed to help reduce energy consumption. The objectives of energy audits are to establish energy audit systems, provide on-site energy audit service and reduce production cost. This study summarized the energy savings implemented in Taiwan's cement industry; the data were obtained from the on-line Energy Declaration System in 2010. The total implemented energy savings amounted to 68,512 kilo liter of crude oil equivalent (KLOE). The energy audit group audited seven Taiwanese cement plants in 2011 and revealed an energy saving potential of 2571.6 MWh of electricity and 1002.8 KLOE of thermal energy. The total potential energy saving was 1708.5 KL of crude oil equivalent (KLOE), equivalent to a 4560 t reduction in CO₂ emissions, representing the annual CO₂ absorption capacity of a 122 ha forest plantation.     

Energy separation in the wake of a cylinder: Effect of Reynolds number and acoustic resonance

Energy separation is a spontaneous redistribution of total energy in a flowing fluid without external work or heat transfer. The energy separation mechanism in the vortex field behind an adiabatic circular cylinder in a cross flow of air is investigated. Time-averaged velocity and temperature measurements taken one diameter downstream of the cylinder (Re ～ 10⁵, M_∞ ～ 0.25) indicate flow reversal. The measured recovery temperature, expressed as distribution of energy separation factor indicates that energy separation is caused by the vortex flow in the wake, enhanced by acoustic excitation, and is insensitive to Reynolds number in the sub critical range studied.     

Life cycle assessment of a Brassica carinata bioenergy cropping system in southern Europe

The energetic and environmental performance of production and distribution of the Brassica carinata biomass crop in Soria (Spain) is analysed using life cycle assessment (LCA) methodology in order to demonstrate the major potential that the crop has in southern Europe as a lignocellulosic fuel for use as a renewable energy source.The Life Cycle Impact Assessment (LCIA) including midpoint impact analysis that was performed shows that the use of fertilizers is the action with the highest impact in six of the 10 environmental categories considered, representing between 51% and 68% of the impact in these categories.The second most important impact is produced when the diesel is used in tractors and transport vehicles which represents between 48% and 77%. The contribution of the B. carinata cropping system to the global warming category is 12.7 g CO₂ eq. MJ⁻¹ biomass produced. Assuming a preliminary estimation of the B. carinata capacity of translocated CO₂ (631 kg CO₂ ha⁻¹) from below-ground biomass into the soil, the emissions are reduced by up to 5.2 g CO₂ eq. MJ⁻¹.The production and transport are as far as a thermoelectric plant of the B. carinata biomass used as a solid fuel consumes 0.12 MJ of primary energy per 1 MJ of biomass energy stored. In comparison with other fossil fuels such as natural gas, it reduces primary energy consumption by 33.2% and greenhouse gas emission from 33.1% to 71.2% depending on whether the capacity of translocated CO₂ is considered or not.The results of the analysis support the assertion that B. carinata crops are viable from an energy balance and environmental perspective for producing lignocellulosic solid fuel destined for the production of energy in southern Europe. Furthermore, the performance of the crop could be improved, thus increasing the energy and environmental benefits.     

Electrohydrodynamic enhancement of in-tube convective condensation heat transfer

An experimental investigation of electrohydrodynamic (EHD) augmentation of heat transfer for in-tube condensation of flowing refrigerant HFC-134a has been performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. The effects of varying the mass flux (55 kg/m² s ⩽ G ⩽ 263 kg/m² s), inlet quality (0.2 ⩽ x_in ⩽ 0.83) and the level of applied voltage (0 kV ⩽ V ⩽ 8 kV) are examined. The heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor 1.5 at the same conditions of the maximum heat transfer enhancement. The improved heat transfer performance and pressure drop penalty are due to flow regime transition from stratified flow to annular flow as has been deduced from the surface temperature profiles along the top and bottom surfaces of the tube.     

Energy saving strategies in air-conditioning for museums

In the museum environment a strict thermal-hygrometric control is necessary primarily for the correct artwork conservation and then for the visitor thermal comfort. Considering that the air-conditioning system has to operate constantly, suitable techniques permit to obtain useful energy savings, allowing, however, a good dynamic microclimatic control.In this paper a case study is presented about various strategies used to reduce energy requirements for HVAC systems in an exhibition room of a modern museum. Using the dynamic simulation code DOE 2.2 and typical climatic hourly data sets, the annual energy use for an all-air system has been calculated, as well as the savings obtainable using different techniques, such as dehumidification by adsorption (desiccant wheel – saving equal to 15% with respect to a base configuration), total energy recovery from the relief air (passive desiccant – 15%), outdoor airflow rate variation (demand control ventilation – 45%). Moreover, the correspondence has been analyzed between the energy request and the admitted variation of indoor temperature and relative humidity: changing the admitted indoor RH range from 50 ± 2% to 50 ± 10%, energy savings around 40% have been obtained. As regards the thermal-hygrometric performance, an optimal control of temperature has been guaranteed with all the configurations, while the best performance in RH control has been obtained with the desiccant system.Considering a simple payback analysis, if the artworks preserved in a museum are particularly sensitive to indoor humidity variation, a desiccant system should be properly used; on the contrary, when the indoor humidity control is not strongly needed, the use of a HVAC system with demand control ventilation is advisable, because of the lowest payback value. The system with total energy recovery presents intermediate features.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Convection heat and mass transfer along a vertical heated plate with film evaporation in a non-Darcian porous medium

A numerical analysis has been carried about to study the heat and mass transfer of forced convection flow with liquid film evaporation in a saturated non-Darcian porous medium. Parametric analyses were conducted concerning the effects of the porosity ε, inlet liquid Reynolds number Re_l, inlet air Reynolds number Re_a on the heat and mass transfer performance. The results conclude that better heat and mass transfer performances are noticed for the system having a higher Re_a, a lower Re_l, and a higher ε. Re_l plays a more important role on the heat and mass transfer performance than Re_a and ε. For the case of ε = 0.4 and Re_a = 10,000, the increases of Nu and Sh for Re_l = 50 are about by 33.9% and 35.3% relative to the values for Re_l = 250.     

Heat transfer enhancement by flow bifurcations in asymmetric wavy wall channels

The enhancement characteristics of heat transfer, through a transition scenario of flow bifurcations, in asymmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element method. The heat transfer characteristics, flow bifurcation and transition scenarios are determined by increasing the Reynolds numbers for three geometrical aspect ratios r = 0.25, 0.375, and 0.5, and Prandtl numbers 1.0 and 9.4. The transition scenarios to transitional flow regimes depend on the aspect ratio. For the aspect ratios r = 0.25 and 0.5, the transition scenario is characterized by one Hopf flow bifurcation. For the aspect ratio r = 0.375, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a periodic flow, and a second Hopf flow bifurcation from a periodic to quasi-periodic flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies ω₁, and ω₁ and ω₂, respectively. For all the aspect ratios and Prandtl numbers, the time-average mean Nusselt number and heat transfer enhancement increases with the Reynolds number as the flow evolves from a laminar to a transitional regime. For both Prandtl numbers, the highest increase in the Nusselt number occurs for the aspect ratio r = 0.5; whereas, the lowest increases happen to r = 0.25. The increase of the Nusselt number occurs at the expense of a higher pumping power, which, for both Prandtl numbers, grows as the aspect ratio increases from r = 0.25 to r = 0.5 for reaching a specific Nusselt number. This enhancement is obtained without the necessity of high volumetric flow rates associated with turbulent flow regimes, which demand much higher pumping powers. Significant heat transfer enhancements are obtained when the asymmetric wavy channel is operated in the appropriate transitional Reynolds number range.     

The effect of porous media particle size on forced convection from a circular cylinder without assuming local thermal equilibrium between phases

A detail numerical analysis of the effect of particle diameter of a packed bed of spherical particles on forced convection about an embedded circular cylinder is presented. This parametric study focusses on the two-phase energy (LTNE—local thermal non-equilibrium) model, which does not assume local thermal equilibrium (LTE) between the solid medium and the fluid. The investigation is performed for a cylinder-to-particle diameter ratio D_cy/d_p = 10–100, at a wide ranges of Reynolds number Re_D = 1–250 and solid-to-fluid thermal conductivity ratio k_r = 0.01–1000. A comparison of predictions from the LTNE and LTE energy models is also made. This paper quantifies the influence of the key non-dimensional parameters on the heat transfer rate. It is also shown that although the presence of the porous materials around the heated cylinder enhances the overall heat transfer and increases the pressure drop in the bed compared to an empty channel, using a porous medium with large particle diameters increases considerably this enhancement in heat transfer and decreases significantly the unfavorable pressure drop.     

Analysis of solar desiccant cooling system for an institutional building in subtropical Queensland,Australia

Institutional buildings contain different types of functional spaces which require different types of heating, ventilating and air conditioning (HVAC) systems. In addition, institutional buildings should be designed to maintain an optimal indoor comfort condition with minimal energy consumption and minimal negative environmental impact. Recently there has been a significant interest in implementing desiccant cooling technologies within institutional buildings. Solar desiccant cooling systems are reliable in performance, environmentally friendly and capable of improving indoor air quality at a lower cost. In this study, a solar desiccant cooling system for an institutional building in subtropical Queensland (Australia) is assessed using TRNSYS 16 software. This system has been designed and installed at the Rockhampton campus of Central Queensland University. The system's technical performance, economic analysis, energy savings, and avoided gas emission are quantified in reference to a conventional HVAC system under the influence of Rockhampton's typical meteorological year. The technical and economic parameters that are used to assess the system's viability are: coefficient of performance (COP), solar fraction, life cycle analysis, payback period, present worth factor and the avoided gas emission. Results showed that, the installed cooling system at Central Queensland University which consists of 10 m² of solar collectors and a 0.400 m³ of hot water storage tank, achieved a 0.7 COP and 22% of solar fraction during the cooling season. These values can be boosted to 1.2 COP and 69% respectively if 20 m² of evacuated tube collector's area and 1.5 m³ of solar hot water storage volume are installed.