Thermal performance of the exhausting and the semi‐exhausting triple‐glazed airflow windows

The thermal performance of the airflow window systems was studied numerically using the finite‐volume method. Effort was directed towards the reduction in space cooling load for the exhausting and the semi‐exhausting triple‐glazed airflow windows. The effects of various parameters such as exhausting airflow rate, solar insolation, and aspect ratio were presented. Some qualitative and quantitative comparisons between two systems were made. It was disclosed that the space‐heat gain was considerably reduced by increasing the exhausting airflow rate, and the decrease in the space‐heat gain of the semi‐exhausting airflow window was larger than that of the exhausting airflow window by about 10 W throughout most of the Re range (except the range of near Re = 0) of this numerical work. Copyright © 2005 John Wiley & Sons, Ltd.     

Fluid flow characteristics and convective heat transfer improvement on a gas turbine blade

The flow field features and heat transfer enhancement are investigated on a gas turbine blade by applying the jet impingement cooling method. The distribution of the flow field and the Nusselt number (Nu) was determined on the targeted surface in the cooling channel. The injection holes of different shapes, such as circular, square, and rectangular were considered. The Reynolds numbers (Re) of the airflow in the range of 2000–5000 and aspect ratios of 0.5–2 were particularly focused. The flow vortices and recirculation in the cooling channel and their influence on the heat transfer enhancement were analyzed in detail under different airflow and geometric conditions. Decreasing the ratio of the distance between jet-to-target plate to the diameter of the jet orifice (H/d) increased the heat transfer rate and produced high-intensity vortices and recirculation zones. It was noticed that the formation and generation of vortices and recirculation have important effects on the convective heat transfer rate at the impingement surface. Local Nusselt number, formation of complex vortices, and airflow recirculation in the cooling channel decreased with the increase in the distance between the jet hole and the targeted surface. It was found that with the increase in the Reynolds number of the jet, heat transfer between cold airflow and the targeted surface increased. Moreover, it was observed that the cooling performance of the round and square jet holes was better than the rectangular holes.     

Numerical simulation of hydrothermal behavior in a concentric curved annular tube

This study performs a numerical investigation of the steady‐state fully developed laminar flow and forced convection heat transfer characteristics in a concentric curved annular tube with two different curvature angles, a 90°‐bend annular tube and a U‐bend annular tube. A wide range of aspect ratios (r* = 0.1, 0.25, 0.5, and 0.75) and three curvature ratios (δ_c = 0.1, 0.2, and 0.5) were adopted in this study. The governing equations consisting of continuity, momentum, and energy equations are solved by considering the outer wall to be insulated (adiabatic), and a constant temperature is applied at the inner wall by using the finite‐volume method (FVM) to investigate the hydrothermal performance for these two different bend angles.Features of axial velocity contours, temperature patterns, and secondary flow streamlines at different cross‐sectional locations along the angular coordinate of curved annulus are observed with a Dean number range of (De = 32‐632). Additionally, the circumferential friction factor and averaged Nusselt number are obtained along the concentric curved annulus flow direction. The numerical results indicate that the normalized average Nusselt number and Performance Evaluation Criteria (PEC) increase with increasing De and curvature ratio for both curvature angles of concentric curved annular tube. Moreover, the normalized average Nusselt number, normalized friction factor‐Reynolds number product, and PEC increase with decreasing the aspect ratio because the annular gap between the surfaces of the inner and outer tubes (the boundaries of annulus) increases with decreasing aspect ratio. The hydrothermal performance of the concentric curved annular tube is higher than that of the straight annular tube attributed to the formation of secondary flows (Dean's vortices) in a cross‐sectional direction and the impact of the inner tube wall boundary. The value of PEC for both curvature angles of the curved annular tube at aspect ratio = 0.1 and De = 632 is approximately two‐fold of the straight annular tube under the same conditions while at aspect ratio = 0.75, it increases by nearly 80%.     

Experimental‐ and numerical‐simulation research on the inner–secondary‐air ratio in a 600‐MWe down‐fired boiler

Using a phase Doppler‐anemometer measurement system, the cold gas/particle‐airflow behavior in a 1:40 scale‐model furnace was assessed to study the influences of adjusting the inner–secondary‐air ratio in a 600‐MW_e multi‐injection and multistaging down‐fired boiler. Numerical simulations were also conducted to verify the results of the modeling trials and to provide heat‐state information. The results demonstrate that reducing the inner–secondary‐air ratio from 19.66% to 7.66% gradually enhances the downward velocity decay of the gas/particle airflow, while the inner secondary‐air downward‐entraining effect on the fuel‐rich flow is weakened. Lowering the inner–secondary‐air ratio greatly inhibits the decay of the near burner–particle volume flux. In addition, the fuel rich–flow ignition distance is reduced, from 1.02 to 0.87 m. A lower inner–secondary‐air ratio is harmful to restrain early NO_x formation. Reducing the ratio also causes the fuel‐rich flow to turn upwards ahead, while the penetration depth of this flow gradually decreases and the maximum temperature in the hopper region falls from 1900 to 1800 K. On the basis of these data, an optimal inner–secondary‐air ratio of 13.66% is recommended.     

Characteristics of heat transfer of impingement system with swap swirl generator

This article presents the computational fluid dynamic simulation of the heat transfer characteristics induced by a swap swirl air‐jet generator on the impingement surface. The study was carried out for conventional and swap twist tape inserts of twisted ratio y = 2.93 with various swap angles (α = 30°, 60°, 90°) at a constant distance of nozzle diameter impingement plate (L = 2D). The results show that the Nusselt number of the swirl impingement air‐jet depends on the twisted tape swap angles and airflow rate. The results also showed that the swap angle of 90° gave notable uniform local heat transfer distribution compared with the typical twist tape and other swap twist tapes (α = 30°, 60°). In addition, the predicted results of the local heat transfer coefficient help explain the local turbulence intensity and generation to assist the industrial applications of swirl impingement air‐cooling jet.     

Linear and nonlinear stability of Soret‐Dufour Lapwood convection near double codimension‐2 points

In this study, the Dufour and Soret effects on natural double‐diffusion convection in a horizontal porous layer was studied numerically using FORTRAN 90 programming and analytically near various convection onset thresholds. The porous layer was subject to a uniform heat and mass fluxes on the horizontal walls while the vertical walls were impermeable and adiabatic. The Darcy model along with the Boussinesq approximation was assumed in the problem formulation. The governing parameters of the problem are the thermal Rayleigh number, R_T, the buoyancy ratio, N, the Lewis number, Le, the aspect ratio of the cavity, A, and the Dufour, D_u, and Soret, S_r, numbers. For a shallow enclosure, the analytical solution was derived assuming zero convection wave number, which is valid near and above criticality. The onset of subcritical, supercritical and oscillatory convection was investigated. Two linear and nonlinear codimension‐2 points were found to exist. Whether the system was subject to constant fluxes and heat and solute, regardless of the aspect ratio of the layer, the subcritical convection behavior remained the same with similarity in the thresholds expressions for subcritical bifurcation.     

Experimental study of exhaust‐gas energy recycling efficiency of hybrid pneumatic power system

A hybrid pneumatic power system (HPPS) comprises an internal combustion engine (ICE), an air compressor, a high‐pressure air storage tank, and a turbine, which stores the flow work instead of a battery's electrochemical energy; moreover, this system can recycle the exhaust‐gas energy and make the ICE operate at its optimal point. Therefore, it can be viewed as a promising solution to increase a system's thermal efficiency and greatly improving exhaust emissions. This paper presents experimental study results concerning the operating capabilities of the HPPS and the effect of the contraction of the cross‐sectional area (CSA) at the merging region of the energy merger pipe for the change in the compressed airflow pressure (P_air) on the exhaust‐gas energy recycling of the HPPS. The experiments were performed on an HPPS that uses an innovative energy merger pipe with a total length of 530 mm, a diameter of 34 mm, and an angle between the two pipes of 30°, and the CSA was adjusted for the change in P_air. The experimental results show that the exhaust‐gas energy recycling and the merger flow energy are significantly dependent on the CSA adjustment for the change in P_air. The optimum conditions for the best merging process can be achieved at a CSA of around 5–35% in the full range of P_air. Under these conditions, the exhaust‐gas energy recycling efficiency reached approximately 75–81%; therefore, a vehicle equipped with an HPPS can achieve efficiency that is approximately 40% higher than that of conventional vehicles. Copyright © 2009 John Wiley & Sons, Ltd.     

An experimental study on the heat recovery from the exhaust air of stenters with a packed bed column

Stenters are the dryers that are commonly used in textile finishing mills. The exhaust air of the stenters is of great potential of energy saving via heat recovery mainly using the latent heat of condensation of the water vapour involved. This study reveals the usage possibility of a packed bed column for the heat recovery from the exhaust air of the stenter. An experimental investigation was carried out with a laboratory‐scale counter flow packed bed column. Additionally, an exergetic evaluation of the system was performed. Water inlet temperature and relative humidity ratio of the exhaust air are the main factors that affect the water outlet temperatures and efficiency of the system. It was observed that a decrease in the water inlet temperature increases the recovered exergy; however, water outlet temperature is not significantly affected by the inlet temperature of the water. The exergy efficiency of the system increases with the increase in the relative humidity ratio of the exhaust air. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of aspect ratio on mixed convection in a horizontal rectangular duct with heated and cooled side walls

This paper describes the effect of aspect ratio on mixed convection in a horizontal rectangular duct with heated and cooled side walls numerically and experimentally. In the numerical analysis, fluid flow and temperature distributions for Ri=1.61, Pr=6.99, Re=100 and aspect ratio, Ar=0.2–10, were obtained by solving dimensionless governing equations using the SIMPLE procedure. The QUICK scheme was applied to the convective term of these equations. In the experimental analysis, the flow behavior for A_r=0.5–2 was visualized by the dye‐injection method. Numerical results showed that the swirl flow was generated along the flow direction, and its pitch length was influenced by A_r. The pitch length was the shortest when A_r=0.5–1, and this tendency was the same in numerical results and experimental results. The heat transfer behavior was also discussed corresponding to the flow, and the heat transfer ratio was highest at A_r=1 in 0.2 ≤ A_r ≤ 10. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20391     

Experimental Investigation of Local Heat Transfer and Skin Friction in Tube Coils

Local heat transfer and skin friction around the tube perimeter of coils were studied in airflow. The heat transfer experiments were performed with two different coils D/d = 23 and 15.6, and skin friction experiments were performed with three different coils D/d = 25, 13.3 and 6.67 In the wide range of Re number from 4×103 till 105 . Variation of the local heat transfer around the perimeter and along the tube was defined. The behavior of the shear stresses at the wall and of the flow modes were studied. Investigations of the heat transfer indicated that with the increase of D/d the difference between heat transfer in the initial thermal section and the stabilized heat transfer increases. Investigations of the shear stress and its fluctuations indicated that, in large-curvature coils, the transition from laminar-vortex flow to turbulent flow around the tube perimeter takes place at different values of Re. In the region of the external generatrix of the bend, the transition occurs at smaller Re, whereas a     

Experimental evaluation of ventilated glazing performance in Hong Kong

Window glazing affects much the indoor environment and the energy use in buildings. While double glazing has better thermal performance than single glazing, the airflow window options carry additional advantage of directly removing the absorbed solar energy in glass panes. This paper reports an experimental study in Hong Kong in evaluating the thermal/energy performance of the above‐mentioned glazing systems. A new approach of using numerical simulation technique to improve the quality of experimental analysis was introduced. Our findings show that the natural‐ventilated glazing system has a better thermal performance than the double‐glazing system since the latter received 13.6% more convective heat gain. The main advantage of the exhaust‐ventilated glazing system lies in the decrease of convective heat transfer to 34.3% of the double glazing, and 19.4% of the single‐glazing types. The results showed that the ventilated glazing schemes in association with daylight utilization could lead to substantial electricity savings in the office environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical Simulation of Natural Convection in a Triangle Cavity Filled with Nanofluids Using Tiwari and Das' Model: Effects of Heat Flux

In this article, we present a fully higher‐order compact (FHOC) finite difference method to investigate the effects of heat flux on natural convection of nanofluids in a right‐angle triangle cavity, where the left vertical side is heated with constant heat flux both partially and throughout the entire wall, the inclined wall is cooled, and the rest of walls are kept adiabatic. The Darcy flow and the Tiwari and Das’ nanofluid models are considered. Investigations with four types of nanofluids were made for different values of Rayleigh numbers with the range of 100 ≤ Ra ≤ 50,000, size of heat flux as 0.1 ≤ ε ≤ 1.0, enclosure aspect ratio as 0.5 ≤ AR ≤ 2.0, and solid volume fraction parameter of nanofluids with the range of 0% ≤ ? ≤ 20%. Results show that the average heat transfer rate increases significantly as particle volume fraction and Rayleigh numbers increase, and the maximum value of average Nusselt number is obtained by decreasing the enclosure aspect ratio. The results also show that the average heat transfer decreases with an increase in the length of the heater. Furthermore, multiple correlations in terms of the Rayleigh numbers and the solid volume fraction of four types of nanoparticles have been established in a general form.     

On the effect of the transversal aspect ratio on the double-glazed solar air heater performance

The transversal aspect ratio of solar air heaters (SAHs) is a critical geometric parameter that influences the heat transfer from the absorber plate to the working fluid and, accordingly, the overall heat loss level. The present work addresses the effect of the aspect ratio on the performance of a solar air heating system and the behavior of heat transfer coefficients (HTCs) within it and along the flow channel. A mathematical model of energy-balance equations was formulated to examine and analyze the double-glazed solar air heater thermal behavior. The Eismann correlation, which is more accurate than Holland's correlation, was employed to determine the HTC between the two glass covers. The useful energy, Nusselt number (Nu), efficiency, overall loss, and HTCs as a function of the aspect ratio were evaluated across the collector length. On the basis of the findings, the higher the ratio, the better the efficiency of the SAH. Indeed, increasing the collector's cross-sectional aspect ratio (r) up to 19 increases useful energy efficiency by more than 87%, Nu by 84%, thermohydraulic efficiency from 0.4 to 0.76, and overall heat loss by 1.15 W/(m² K). Furthermore, reducing r from 19 to 2 will improve the collector power from 1.855 to 3.473 kW.     

Thermo‐economic investigation: an insight tool to analyze NGCC with calcium‐looping process and with chemical‐looping combustion for CO2 capture

In this work, three kinds of natural gas‐based power generation processes for CO₂ capture and storage, that is, natural gas‐combined cycle with pre‐combustion decarburization (NGCC‐PRE), NGCC‐PRE with calcium‐looping process, and NGCC‐PRE with chemical‐looping combustion (NGCC‐CLC), are analyzed by Aspen Plus. The effects of two decisive variables (i.e., steam‐to‐natural gas (S/NG) ratio and oxygen‐to‐natural gas (O/NG) ratio) on the thermodynamic performances of individual process, such as energy and exergy efficiencies, are investigated systematically. Based on simulation outcomes, all the three processes are favored by operating at S/NG = 2.0 and O/NG = 0.65. Furthermore, comparisons of individual system efficiencies and exergy destruction contributor are herein involved. The results show that the highest system efficiencies and lowest exergy destruction are achieved in the NGCC‐CLC process. In addition, capital investment, dynamic payback period, net present value, and internal rate of return are used for deciding the economic feasibility and surely are involved in this work for comparison purpose. Copyright © 2016 John Wiley & Sons, Ltd.     

Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

Heat pipes and two-phase thermosyphon systems are passive heat transfer systems that employ a two-phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (10⁵ W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air-conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air-to-air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m³/h, evaporator inlet-air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (C_e/C_c) of 1 and 1.5. For HPHE's steady-state operation, a mathematical model for heat-transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet-air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (C_e/C_c) of 1.5. The predicted and experimental values of condenser outlet-air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet-air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.     

Performance evaluation of a single solar air heater with N‐subcollectors connected in different combinations

In the present communication, a generalized procedure has been described for the performance evaluation of a solar air heater with N‐subcollectors with identical collector aspect ratio connected in various combinations such as in series, parallel and series–parallel. It has been found that the performance of a solar air heating system can be improved by operating several subcollectors in series in place of a single large collector with the same total area. Results have been presented to show the effect of various design parameters, viz. collector aspect ratio, mass flow rate, etc. on the performance of solar air heater with N‐subcollectors in series, parallel and series–parallel combinations. Copyright © 1999 John Wiley & Sons, Ltd.     

Influence of high rates of supplemental cooled EGR on NOx and PM emissions of an automotive HSDI diesel engine using an LP EGR loop

Previous experimental studies on diesel engine have demonstrated the potential of exhaust gas recirculation (EGR) as an in‐cylinder NO_x control method. Although an increase in EGR at constant boost pressure (substitution EGR) is accompanied with an increase in particulate matter (PM) emissions in the conventional diesel high‐temperature combustion (HTC), the recirculation of exhaust gases supplementary to air inlet gas (supplemental EGR) by increasing the boost pressure has been suggested as a way to reduce NO_x emissions while limiting the negative impact of EGR on PM emissions. In the present work, a low‐pressure (LP) EGR loop is implemented on a standard 2.0 l automotive high‐speed direct injection (HSDI) turbocharged diesel engine to study the influence of high rates of supplemental cooled EGR on NO_x and PM emissions. Contrary to initial high‐pressure (HP) EGR loop, the gas flow through the turbine is unchanged while varying the EGR rate. Thus, by closing the variable geometry turbine (VGT) vanes, higher boost pressure can be reached, allowing the use of high rates of supplemental EGR. Furthermore, recirculated exhaust gases are cooled under 50°C and water vapour is condensed and taken off from the recirculated gases. An increase in the boost pressure at a given inlet temperature and dilution ratio (DR) results in most cases an increase in NO_x emissions and a decrease in PM emissions. The result of NO_x–PM trade‐off, while varying the EGR rate at fixed inlet temperature and boost pressure depends on the operating point: it deteriorates at low load conditions, but improves at higher loads. Further improvement can be obtained by increasing the injection pressure. A decrease by approximately 50% of NO_x emissions while maintaining PM emission level, and brake specific fuel consumption can be obtained with supplemental cooled EGR owing to an LP EGR loop, compared with the initial engine configuration (HP moderately cooled EGR). Copyright © 2008 John Wiley & Sons, Ltd.     

A simplified method for modelling the effect of blinds on window thermal performance

An approximate method is presented for predicting the effect of a louvered blind on the centre‐glass thermal performance of a fenestration. The method combines a one‐dimensional heat transfer model with data from a numerical simulation of the window and blind. Sample results for a blind mounted on the indoor surface of a window show the effect of blind slat angle on heat transmission. Both summer and winter conditions are considered. The results show that a louvered blind can improve the U‐value of a standard double‐glazed window by up to 37%. Also, the radiation heat exchange with the room can be dramatically reduced (by up to 60%), which will improve the level of occupant comfort. However, there was found to be a trade‐off between U‐value and occupant comfort; placing the blind closer to the window improves the U‐value, but increases the radiation heat exchange with the room. The predictions from the present simplified method compare well with results from a full two‐dimensional computational fluid dynamics solution of the conjugate blind/window interaction. Copyright © 2005 John Wiley & Sons, Ltd.     

Heat transfer regulation in a rectangular enclosure using a rotating plate

The effects of an inner rotating plate with horizontal axis on the heat transfer in a differentially heated vertical enclosure were investigated experimentally. The aspect ratio of the enclosure height/width was 1 throughout the experiments. An acrylic plate with a small thermal conductivity was installed horizontally at the center of the square enclosure, and was rotated at various speeds for normal and reverse rotations by using the motor attached outside of the enclosure. Purified water was used as the working fluid. The flow pattern was sketched by a visualization experiment using aluminum powder. The heat transfer results were also compared with those from a previous paper on a rotating cylinder. It is clarified here that the heat transfer rate of the enclosure depends largely on the parameter Gr_w/Re_ω², and is characterized by three regions. The heat transfer rate of the enclosure with a rotating plate is somewhat larger than that of a rotating cylinder in the forced convection region. The rotating plate used here will be useful for regulation of wide‐ranging heat transfer. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 342–353, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10099     

Experimental and numerical study on charging processes of an ice‐on‐coil thermal energy storage system

In this study, an external melt ice‐on‐coil thermal storage was studied and tested over various inlet conditions of secondary fluid—glycol solution—flow rate and temperature in charging process. Experiments were conducted to investigate the effect of inlet conditions of secondary fluid and validate the numerical model predictions on ice‐on‐coil thermal energy storage system. The total thermal storage energy and the heat transfer rate in the system were investigated in the range of 10 l min ^?1?V??60 l min ^?1. A new numerical model based on temperature transforming method for phase change material (PCM) described by Faghri was developed to solve the problem of the system consisting of governing equations for the heat transfer fluid, pipe wall and PCM. Numerical simulations were performed to investigate the effect of working conditions of secondary fluid and these were compared with the experimental results. The numerical results verified with experimental investigation show that the stored energy rises with increasing flow rate a decreasing tendency. It is also observed that the inlet temperature of the fluid has more influence on energy storage quantity than flow rate. Copyright © 2006 John Wiley & Sons, Ltd.