Thermal behavior of buildings with curved roofs as compared with flat roofs

In this paper, a detailed finite element model dealing with heat transfer through a domed or vaulted roof is suggested based on a three-dimensional heat transfer equation and solar geometry. This model allows a comparison of the thermal behavior of curved and flat roofs in terms of heat flux and daily heat flow through them into an air-conditioned building under different climatic conditions. The results of numerical calculations show that the ratio of daily heat flow through curved roofs to that through flat ones is not affected by the curve radius, thickness and construction material of the roof, but is significantly influenced by the half rim angle θ₀ of the roofs and the ambient temperature. Compared to a flat roof, under typical hot dry climatic conditions, the daily heat flow through a domed roof of θ₀=90° is about 40% higher, whereas the daily heat flow through a south–north oriented and an east–west oriented vault of θ₀=90° is about 20 and 27% higher, respectively. The reason for this is mainly attributed to the convective heat transfer between the enlarged curved roof and ambient air. However, when θ₀<50°, heat flux and daily heat flow through a curved roof is close to that through a flat roof. The results also confirm that curved roofs are not suitable for areas with higher air temperature and intense sky diffuse radiation typical of hot humid areas.     

Design and performance of roofs

The factors influencing roof design with respect to energy conservation are surveyed. In particular low-pitched (～15° inclination) roofs have been shown to possess thermal insulation advantages. Thus relatively low-pitched, northerly facing roofs, together with 61° pitch (i.e. the optimal inclination of plane, solar-energy collectors for maximum winter gain in UK latitudes) southerly facing roofs will probably become more commonplace.     

Thermal performance analysis of pneumatic structures

Pneumatic structures involve inflatable membranes using air as the supporting medium. These membranes have wide applications to port structures, exhibition structures, disaster shelters, etc. The thermal performance characteristics of pneumatic structures have been studied by solving the appropriate energy-balance equations. The effects of spraying with water and painting the structure with a compound having good reflective characteristics have been studied.     

Thermal performance of dimpled surfaces in laminar flows

The present study provides data which illustrate the effects of an array of dimples on local and spatially-averaged surface Nusselt number distributions, as well as on friction factors in channels with laminar flow. Trends of spatially-averaged Nusselt numbers and friction factors are provided as they vary with dimple depth, channel height, Reynolds number from 260 to 1030, and the use of protrusions on the opposite channel wall. When compared with turbulent flow results, the present laminar data illustrate changes due to the absence of turbulence transport. For example, in contrast to turbulent flows, the present laminar flow data show that there is no overall benefit from the use of a top wall with protrusions. In addition, spatially-averaged Nusselt number ratios and friction factor ratios measured on a deep dimpled surface with a smooth top wall show trends which are opposite from ones observed in turbulent flows, since lower laminar heat transfer augmentations are present for smaller channel heights when compared at the same Reynolds number.     

Thermal analysis in fluidized bed drying of moist particles

This paper deals with thermal modeling of the fluidized bed drying of wet particles to study heat and mass transfer aspects and drying thermal efficiencies. The model is then validated with the literature experimental data obtained for corn. A parametric investigation is undertaken to study the effects of the inlet air temperature, the air velocity and the initial moisture content of the material (i.e. corn) on the process thermal efficiency. The results show that the thermal efficiencies of the fluidized bed drying decrease sharply with decreasing moisture content of corn and hence increasing drying time, and apparently become the lowest at the end of the drying process. This clearly indicates that the moisture transfer from the material depends strongly on the air temperature, air velocity and the moisture content of material. A good agreement is obtained between the model predictions and the available experimental results.     

Enhancement of PCM melting in enclosures with horizontally-finned internal surfaces

A numerical model for simulating the melting of a phase change material (PCM) housed within an internally-finned metal enclosure is developed. A finite volume approach, utilizing the temperature-transforming model for phase change, is used to predict the conjugate heat transfer in the cavity walls and fins, as well as within the molten PCM. The influence of the number of fins, the fin length and thickness, and the hot wall temperature on the melting process is reported. With horizontal fins, rapid melting occurs during the early stages of the phase change, followed by a second, slow melting regime. Analytical correlations are developed that can be used to quickly estimate melting rates during both melting regimes, and it is shown that the predictions of the correlations are in good agreement with those of the detailed model.     

Aerodynamic and aeroacoustic performance of airfoils with morphing structures

Aerodynamic and aeroacoustic performance of airfoils fitted with morphing trailing edges are investigated using a coupled structure/fluid/noise model. The control of the flow over the surface of an airfoil using shape optimization techniques can significantly improve the load distribution along the chord and span lengths whilst minimising noise generation. In this study, a NACA 63‐418 airfoil is fitted with a morphing flap and various morphing profiles are considered with two features that distinguish them from conventional flaps: they are conformal and do not rely on conventional internal mechanisms. A novel design of a morphing flap using a zero Poisson's ratio honeycomb core with tailored bending stiffness is developed and investigated using the finite element model. Whilst tailoring the bending stiffness along the chord of the flap yields large flap deflections, it also enables profile tailoring of the deformed structure which is shown to significantly affect airfoil noise generation. The aeroacoustic behaviour of the airfoil is studied using a semi‐empirical airfoil noise prediction model. Results show that the morphing flap can effectively reduce the airfoil trailing edge noise over a wide range of flow speeds and angles of attack. It is also shown that appropriate morphing profile tailoring improves the effect of morphing trailing edge on the aerodynamic and aeroacoustic performance of the airfoil. Copyright © 2015 John Wiley & Sons, Ltd.     

Vortices and vortical structures in internal aerodynamics

The paper aims at summarizing the author‘s recent phenomenological study of the origin,development and identification of vortical structures in internal aerodynamics.A connection between evolution of these structures and flow separation in closed curved channels is also discussed.It has been shown that in real fluids the individual vortex cores very sonn lose their identity and merge into a new dissipative structure,the properties of which still have to be defined.     

Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.     

Thermal performance of linear solar concentrators with tubular absorbers

The outlet temperature, corresponding thermal efficiency and the stagnation temperature available with a linear solar concentrator using a tubular absorber have been calculated, taking into account the temperature dependence of the heat-loss coefficient. The results of some typical numerical calculations are presented graphically and discussed.     

Analysis of turbulent heat transfer and fluid flow in channels with various ribbed internal surfaces

This paper presents results of a numerical investigation of heat transfer and flow pattern characteristics of a channel with repeated ribs on one broad wall. Numerical computations are performed for seven ribs placed on the bottom wall of a channel for Reynolds numbers ranging from 10,000 to 30,000. The newly modified ribs (the ones with convex pointing upstream/downstream rib, wedge pointing upstream/downstream rib, concave pointing upstream/downstream rib and also concave-concave rib as well as convex-concave rib), are proposed for simulation with prospect to reduce flow separation and extend reattachment area compared to the unmodified square rib. The numerical results are reported in forms of flow structure, temperature field, turbulent kinetic energy, Nusselt number, friction factor and thermal enhancement factor. The results indicate the rib with concave-concave surfaces efficiently suppresses flow separation bubble in the corner of the rib and induces large recirculation zone over those of the others, hence giving the highest Nusselt number and friction factor. On the other hand, the one with convex-concave surface provides the lowest friction factor with moderate Nusselt number. Due to the prominent effect of its low friction factor, the rib with convex-concave surface offers the highest thermal enhancement factor of 1.19.     

Thermal contact resistance between plasma-sprayed particles and flat surfaces

Plasma-sprayed molybdenum and yttria-stabilized zirconia particles (38–63 μm diameters) were sprayed onto glass and Inconel 625 held at either room temperature or 400 °C. Samples of Inconel 625 were also preheated for 3 h, and then air-cooled to room temperature before spraying. Photographs of the splats were captured by using a fast charge-coupled device (CCD) camera. A rapid two-color pyrometer was used to collect thermal radiation from the particles during flight and spreading to follow the evolution of their temperature. The temperature evolution was used to determine the cooling rate of spreading particles. An analytical heat conduction model was developed to calculate the thermal contact resistance at the interface of the plasma-sprayed particles and the surfaces from splat cooling rates. The analysis showed that thermal contact resistance between the heated or preheated surfaces and the splats was more than an order of magnitude smaller than that on non-heated surfaces held at room temperature. Particles impacting on the heated or preheated surfaces had cooling rates that were significantly larger than those on surfaces held at room temperature, which was attributed to smaller thermal contact resistance.     

Enhanced performance of counter flow SOFC with partial internal reformation

We study a counter-flow solid oxide fuel cell system and consider the challenges faced in minimizing thermal variations from the nominal operating conditions for a reasonable range of power tracking. Blower dynamics, reformer transport delays, spatial distribution of the heat generated and the resulting thermal response are among the issues considered. A novel approach, relying on partial internal reformation of the feedstock is proposed as a remedy to maintain a strong level of power tracking with minimal thermal stress to the fuel cell.     

Thermal instability in a plane channel with internal heating and horizontal Poiseuille throughflow

The effect of a basic Poiseuille throughflow on the thermal instability of a horizontal fluid layer bounded by two plane parallel walls is studied. An unstable thermal stratification is studied, entirely due to a uniform internal heat generation in the fluid, whereas the thermal boundary conditions do not impress any temperature difference across the fluid layer. Two cases are investigated: a symmetric case where both boundaries are perfectly conducting; a non-symmetric case where the lower boundary is adiabatic and the upper boundary is perfectly conducting. A linear stability analysis is carried out and the eigenvalue problem is solved numerically for arbitrary oblique rolls, and by a symbolic weighted residual method in the special case of longitudinal rolls. The main result is that the basic Poiseuille flow does not influence the thermoconvective instability at the onset of the least stable modes, i.e. the longitudinal rolls. Thus, the critical conditions are just the same as for a fluid at rest in the basic state. Although the focus is on the thermoconvective instability, it is proved that, even in the presence of the internal heat generation, Squire’s theorem holds for the hydrodynamic instability of the plane Poiseuille flow.     

Thermal performance and energy requirements of buildings

The need for simple yet accurate techniques for assessing the thermal performances of buildings is discussed, particularly in relation to residential buildings in Australia. A method is presented for calculating heating or cooling load requirements in terms of indoor temperatures in totally unconditioned (i.e. free-running) buildings. The method accounts for thermal capacitance in the building fabric and intermittent heating or cooling. It can also allow for climatic variability from day to day. The method is a generalization of the variable base degree-day technique which has recently achieved prominence in the United States of America.     

Thermal conductivity of thin single-crystalline germanium-on-insulator structures

We report on the effective cross-plane thermal conductivity of single-crystal Ge layers from 42 to 100 nm on a SiO₂/Si substrate in the temperature range 30–300 K. We observe a drastic reduction of the thermal conductivity compared to bulk germanium. A large contribution to the temperature rise in the Ge layer is due to the interfacial thermal resistance between c-Ge/SiO₂. The use of a size-dependent intrinsic thermal conductivity of the Ge layer instead of the bulk thermal conductivity improves the consistency with values of the thermal boundary resistance derived from the diffusive mismatch model. Ultrathin films of Ge suffer from a lower reduction of the thermal conductivity compared to ultrathin films of Si, which makes germanium-on-insulator structures promising candidates for devices with reduced self-heating effects compared to silicon-on-insulator structures.     

Thermal performance study of integrated cold plate with power module

A novel cold plate for cooling of the electronic components with high heat flux and high heat dissipation requirements is proposed. The cold-plate structure of the S-type with guide plates is introduced to avoid the heat hot concentration and increase the heat transfer area. The experimental results show that the maximum chip temperature of the novel cold plate is approximately 40% lower than those of the conventional cold plate. Thermal performance optimizations are conducted, indicating that it is extremely effective to install the heat source on two sides of the cold plate. Compared with the single-side heat source, the maximum chip temperature is increased only 20%. However, the heat dissipation is doubled in the limited space for the double-sides arrangement heat source. Moreover, the integrated density of the power module is greatly enhanced by using the cold plate. Transient state temperature variation indicates that the cold plate have quick thermal response in start-up process. It is beneficial to the heat dissipation of integrated module for high power density.     

Thermal performance characteristics of STC system with Phase Change Storage

Storing solar energy heat using Phase Change Materials (PCM) is an effective method. The combination of solar collector and PCM in one unit is being currently studied. The performance characteristics of the proposed Solar Tube Collector (STC) are being analysed analytically and experimentally. Fundamental experiments were performed to simulate a direct contact solar storage system, using two vertical cylindrical concentric tubes with the annular space between them filled Stearic acid (C₁₈H₃₈O₂, melting temperature 70 °C). Experimental testing apparatus has been set up to simulated real system conditions, for an assumed initial and boundary conditions, to provide quantitative information concerning the heat transfer and the timewise evolution of the solid-liquid interface and to identify the role and pattern of natural convection and of the movement of the boundary layer in the liquid phase. For the heat charging mode, the experimental results for different types of fin structures have shown that the effect of melting process is strongly effected by the variation of the imposed conditions, in addition to the different trends of the melting profiles along the axial direction due to the effect of natural convection.     

Forced convection and flow friction characteristics of air-cooled horizontal equilateral triangular ducts with ribbed internal surfaces

Experimental studies have been conducted to examine the forced convection and flow friction characteristics of air-cooled horizontal equilateral triangular ducts whose internal surfaces have been fabricated with uniformly spaced square ribs. Effects of duct geometry (i.e. relative rib height (H/D) and relative rib-to-rib spacing (S/W)) as well as the hydraulic-diameter based Reynolds number (Re_D) on heat transfer coefficient and friction factor of a fully developed turbulent air flow in a horizontal triangular duct with ribbed internal surfaces have been fully investigated. The ranges of experimental parameters under consideration are: H/D from 0.11 to 0.21, S/W from 3.41 to 13.93, and Re_D from 4000 to 23,000. Optimum relative rib height and relative rib-to-rib spacing corresponding to maximum thermal performance of this system have been determined, which are equal to 0.18 and 7.22, respectively. Flow friction in the triangular duct increases rather linearly with the relative rib height, but a maximum flow friction factor is obtained at the relative rib-to-rib spacing of 7.22. Non-dimensional expressions for prediction of average Nusselt number and friction factor in terms of Re_D, H/D and S/W have been developed correspondingly, which correlate well with the experimental data with maximum deviations of ±3.5% and ±8.7%, respectively.