Energy efficiency with natural ventilation: a case study

Application of laboratory analogue modelling of air flow in a naturally ventilated shopping mall is reviewed in this paper. A detailed study of the ventilation was undertaken to establish the principles and to explore how natural ventilation might interact with a localised mechanical ventilation system designed to enhance the cooling of a high density food court area. The case study is used to show how the combination of laboratory modelling and simplified mathematical modelling enables one to rapidly identify the various flow regimes which can occur, to quantify the resultant flows and mean temperatures and to thereby develop appropriate ventilation strategies for the different external conditions which occur through the year.

High impulsivity predicting vulnerability to cocaine addiction in rats: some relationship with novelty preference but not novelty reactivity, anxiety or stress.

RATIONALE: Impulsivity is a vulnerability marker for drug addiction in which other behavioural traits such as anxiety and novelty seeking ('sensation seeking') are also widely present. However, inter-relationships between impulsivity, novelty seeking and anxiety traits are poorly understood. OBJECTIVE: The objective of this paper was to investigate the contribution of novelty seeking and anxiety traits to the expression of behavioural impulsivity in rats. METHODS: Rats were screened on the five-choice serial reaction time task (5-CSRTT) for spontaneously high impulsivity (SHI) and low impulsivity (SLI) and subsequently tested for novelty reactivity and preference, assessed by open-field locomotor activity (OF), novelty place preference (NPP), and novel object recognition (OR). Anxiety was assessed on the elevated plus maze (EPM) both prior to and following the administration of the anxiolytic drug diazepam, and by blood corticosterone levels following forced novelty exposure. Finally, the effects of diazepam on impulsivity and visual attention were assessed in SHI and SLI rats. RESULTS: SHI rats were significantly faster to enter an open arm on the EPM and exhibited preference for novelty in the OR and NPP tests, unlike SLI rats. However, there was no dimensional relationship between impulsivity and either novelty-seeking behaviour, anxiety levels, OF activity or novelty-induced changes in blood corticosterone levels. By contrast, diazepam (0.3-3 mg/kg), whilst not significantly increasing or decreasing impulsivity in SHI and SLI rats, did reduce the contrast in impulsivity between these two groups of animals. CONCLUSIONS: This investigation indicates that behavioural impulsivity in rats on the 5-CSRTT, which predicts vulnerability for cocaine addiction, is distinct from anxiety, novelty reactivity and novelty-induced stress responses, and thus has relevance for the aetiology of drug addiction.

A detailed chemistry multi-cycle simulation of a gasoline fueled HCCI engine operated with NVO

A previously developed Stochastic Reactor Model (SRM) is used to simulate combustion in a four cylinder in-line four-stroke naturally aspirated direct injection Spark Ignition (SI) engine modified to run in Homogeneous Charge Compression Ignition (HCCI) mode with a Negative Valve Overlap (NVO). A portion of the fuel is injected during NVO to increase the cylinder temperature and enable HCCI combustion at a compression ratio of 12:1. The model is coupled with GT-Power, a one-dimensional engine simulation tool used for the open valve portion of the engine cycle. The SRM is used to model in-cylinder mixing, heat transfer and chemistry during the NVO and main combustion. Direct injection is simulated during NVO in order to predict heat release and internal Exhaust Gas Recycle (EGR) composition and mass. The NOx emissions and simulated pressure profiles match experimental data well, including the cyclic fluctuations. The model predicts combustion characteristics at different fuel split ratios and injection timings. The effect of fuel reforming on ignition timing is investigated along with the causes of cycle to cycle variations and unstable operation. A detailed flux analysis during NVO unearths interesting results regarding the effect of NOx on ignition timing compared with its effect during the main combustion. © 2009 SAE International.

Modelling nonlinear vehicle dynamics with neural networks

The nonlinear modelling ability of neural networks has been widely recognised as an effective tool to identify and control dynamic systems, with applications including nonlinear vehicle dynamics which this paper focuses on using multi-layer perceptron networks. Existing neural network literature does not detail some of the factors which effect neural network nonlinear modelling ability. This paper investigates into and concludes on required network size, structure and initial weights, considering results for networks of converged weights. The paper also presents an online training method and an error measure representing the network's parallel modelling ability over a range of operating conditions. Copyright © 2010 Inderscience Enterprises Ltd.

Surface-waves, stability, and scattering for a lined duct with flow

We consider a straight cylindrical duct with a steady subsonic axial flow and a reacting boundary (e.g. an acoustic lining). The wave modes are separated into ordinary acoustic duct modes, and surface modes confined to a small neighbourhood of the boundary. Many researchers have used a mass-spring-damper boundary model, for which one surface mode has previously been identified as a convective instability; however, we show the stability analysis used in such cases to be questionable. We investigate instead the stability of the surface modes using the Briggs-Bers criterion for a Flügge thin-shell boundary model. For modest frequencies and wavenumbers the thin-shell has an impedance which is effectively that of a mass-spring-damper, although for the large wavenumbers needed for the stability analysis the thin-shell and mass-spring-damper impedances diverge, owing to the thin shell's bending stiffness. The thin shell model may therefore be viewed as a regularization of the mass-spring-damper model which accounts for nonlocally-reacting effects. We find all modes to be stable for realistic thin-shell parameters, while absolute instabilities are demonstrated for extremely thin boundary thicknesses. The limit of vanishing bending stiffness is found to be a singular limit, yielding absolute instabilities of arbitrarily large temporal growth rate. We propose that the problems with previous stability analyses are due to the neglect of something akin to bending stiffness in the boundary model. Our conclusion is that the surface mode previously identified as a convective instability may well be stable in reality. Finally, inspired by Rienstra's recent analysis, we investigate the scattering of an acoustic mode as it encounters a sudden change from a hard-wall to a thin-shell boundary, using a Wiener-Hopf technique. The thin-shell is considered to be clamped to the hard-wall. The acoustic mode is found to scatter into transmitted and reflected acoustic modes, and surface modes strongly linked to the solid waves in the boundary, although no longitudinal or transverse waves within the boundary are excited. Examples are provided that demonstrate total transmission, total reflection, and a combination of the two. This thin-shell scattering problem is preferable to the mass-spring-damper scattering problem presented by Rienstra, since the thin-shell problem is fully determined and does not need to appeal to a Kutta-like condition or the inclusion of an instability in order to avoid a surface-streamline cusp at the boundary change.

Monte Carlo filtering and smoothing with application to time-varying spectral estimation

We develop methods for performing filtering and smoothing in non-linear non-Gaussian dynamical models. The methods rely on a particle cloud representation of the filtering distribution which evolves through time using importance sampling and resampling ideas. In particular, novel techniques are presented for generation of random realisations from the joint smoothing distribution and for MAP estimation of the state sequence. Realisations of the smoothing distribution are generated in a forward-backward procedure, while the MAP estimation procedure can be performed in a single forward pass of the Viterbi algorithm applied to a discretised version of the state space. An application to spectral estimation for time-varying autoregressions is described.

Sound transmission in strongly-curved slowly-varying cylindrical and annular lined ducts with flow

In this paper we consider the propagation of acoustic waves along a curved hollow or annular duct with lined walls. The curvature of the duct centreline and the wall radii vary slowly along the duct, allowing application of an asymptotic multiple scales analysis. This generalises Rienstra's analysis of a straight duct of varying cross-sectional radius. The result of the analysis is that the modal wavenumbers and mode shapes are determined locally as modes of a torus with the same local curvature, while the amplitude of the modes evolves as the mode propagates along the duct. The duct modes are found numerically at each axial location using a pseudo-spectral method. Unlike the case of a straight duct, there is a fundamental asymmetry between upstream and downstream propagating modes, with some mode shapes tending to be concentrated on either the inside or outside of the bend depending on the direction of propagation. The interaction between the presence of wall lining and curvature is investigated in particular; for instance, in a representative case it is found that the curvature causes the first few acoustic modes to be more heavily damped by the duct boundary than would be expected for a straight duct. Analytical progress can be made in the limit of very high mode order, in which case well-known 'whispering gallery' modes, localised close to the wall, can be identified.

Measurements of thermal radiation in ultralight metal foams with open cells

Highly porous ultralightweight cellular metal foams with open cells have attractive mechanical, thermal, acoustic and other properties and are currently being exploited for high-temperature applications (e.g. acoustic liners for combustion chambers). In such circumstances, thermal radiation in the metal foam becomes a significant mechanism of heat transfer. This paper presents results from experimental measurements on radiative transfer in Fe-Cr-Al-Y (a steel-based high-temperature alloy) foams having high porosity (95 per cent) and different cell sizes, manufactured at low cost from the sintering route. The spectral transmittance and reflectance are measured at different infrared wavelengths ranging from 2.5 to 50 μm, which are subsequently used to determine the extinction coefficient and foam emissivity. The results show that the spectral quantities are strongly dependent on the wavelength, particularly in the short-wavelength regime (less than 25 μm). While the extinction coefficient decreases with increasing cell size, the effect of cell size on foam reflectance is not significant. When the temperature is increased, the total extinction coefficient increases but the total reflectance decreases. The effective radiative conductivity of the metal foam is obtained by using the guarded hot-plate apparatus. With the porosity fixed, the effective radiative conductivity increases with increasing cell size and increasing temperature. © IMechE 2004.

Natural convection in metal foams with open cells

This paper presents a combined experimental and numerical study on natural convection in open-celled metal foams. The effective thermal conductivities of steel alloy (FeCrAlY) samples with different relative densities and cell sizes are measured with the guarded-hot-plate method. To examine the natural convection effect, the measurements are conducted under both vacuum and ambient conditions for a range of temperatures. The experimental results show that natural convection is very significant, accounting for up to 50% of the effective foam conductivity obtained at ambient pressure. This has been attributed to the high porosity (ε > 0.9) and inter-connected open cells of the metal foams studied. Morphological parameters characterizing open-celled FeCrAlY foams are subsequently identified and their cross-relationships are built. The non-equilibrium two-equation energy transfer model is employed, and selected calculations show that the non-equilibrium effect between the solid foam skeleton and air is significant. The study indicates that the combined parameter, i.e., the porous medium Rayleigh number, is no longer appropriate to correlate natural convection by itself when the Darcy number is sufficiently large as in the case of natural convection in open-celled metal foams. Good agreement between model predictions and experimental measurements is obtained. © 2005 Elsevier Ltd. All rights reserved.

Convective heat dissipation with lattice-frame materials

This paper presents experimental results on heat transfer and pressure drop for a compact heat sink made of fully triangulated, lightweight (porosity∼0.938), aluminum lattice-frame materials (LFMs). Due to the inherent structural anisotropy of the LFMs, two mutually perpendicular orientations were selected for the measurements. Constant heat flux was applied to the heat sink under steady state conditions, and dissipated by forced air convection. The experimental data were compared with those predicted from an analytical model based on fin analogy. The experimental results revealed that pressure drop is strongly dependent upon the orientation of the structure, due mainly to the flow blockage effect. For heat transfer measurements, typical local temperature distributions on the substrate under constant heat flux conditions were captured with infrared camera. The thermal behavior of LFMs was found to follow closely that of cylinder banks, with early transition Reynolds number (based on strut diameter) equal to about 300. The Nusselt number prediction from the fin-analogy correlates well with experimental measurements, except at low Reynolds numbers where a slightly underestimation is observed. Comparisons with empty channels and commonly used heat exchanger media show that the present LFM heat sink can remove heat approximately seven times more efficient than an empty channel and as efficient as a bank of cylinders at the same porosity level. The aluminum LFMs are extremely stiff and strong, making them ideal candidates for multifunctional structures requiring both heat dissipation and mechanical load carrying capabilities. © 2003 Elsevier Ltd. All rights reserved.

Thermal radiation in ultralight metal foams with open cells

This paper presents results from experimental measurements on radiative transfer in FeCrAlY (a steel based high temperature alloy) foams having high porosity (95%) and different cell sizes, manufactured at low cost from the sintering route. The spectral transmittance and reflectance are measured at different infrared wavelengths ranging from 2.5 to 50 μm, which are subsequently used to determine the extinction coefficient and foam emissivity. The results show that the spectral quantities are strongly dependent on the wavelength, particularly in the short wavelength regime (<25 μm). Whilst the extinction coefficient decreases with increasing cell size, the effect of cell size on foam reflectance is not significant. When the temperature is increased, the total extinction coefficient increases but the total reflectance decreases. An analytical model based on geometric optics laws, diffraction theory and metal foam morphology is developed to predict the radiative transfer, with cell size (or cell ligament diameter) and porosity identified as the two key parameters that dictate the foam radiative properties. Close agreement between the predicted effective foam conductivity due to radiation alone and that measured is observed. At fixed porosity, the radiative conductivity of the metal foam increases with increasing cell size and temperature. © 2004 Elsevier Ltd.All rights reserved.

Mechanical and magnetic properties of metal fibre networks, with and without a polymeric matrix

Bonded networks of metal fibres are highly porous, permeable materials, which often exhibit relatively high strength. Material of this type has been produced, using melt-extracted ferritic stainless steel fibres, and characterised in terms of fibre volume fraction, fibre segment (joint-to-joint) length and fibre orientation distribution. Young's moduli and yield stresses have been measured. The behaviour when subjected to a magnetic field has also been investigated. This causes macroscopic straining, as the individual fibres become magnetised and tend to align with the applied field. The modeling approach of Markaki and Clyne, recently developed for prediction of the mechanical and magneto-mechanical properties of such materials, is briefly summarised and comparisons are made with experimental data. The effects of filling the inter-fibre void with compliant (polymeric) matrices have also been explored. In general the modeling approach gives reliable predictions, particularly when the network architecture has been characterised using X-ray tomography. © 2005 Published by Elsevier Ltd.