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d Ruivo, C.R.a b, Vaz D. C. c. "Prediction of the heat gain of external walls: An innovative approach for full-featured excitations based on the simplified method of Mackey-and-Wright." Applied Energy. 155 (2015): 378-392. AbstractWebsite

Nowadays, simulation tools are available for calculating the thermal loads of multiple rooms of buildings, for given inputs. However, due to inaccuracies or uncertainties in some of the input data (e.g., thermal properties, air infiltrations flow rates, building occupancy), the evaluated thermal load may represent no more than just an estimate of the actual thermal load of the spaces. Accordingly, in certain practical situations, simplified methods may offer a more reasonable trade-off between effort and results accuracy than advanced software. Hence, despite the advances in computing power over the last decades, simplified methods for the evaluation of thermal loads are still of great interest nowadays, for both the practicing engineer and the graduating student, since these can be readily implemented or developed in common computational-tools, like a spreadsheet.The method of Mackey and Wright (M&W) is a simplified method that upon values of the decrement factor and time lag of a wall (or roof) estimates the instantaneous rate of heat transfer through its indoor surface. It assumes cyclic behaviour and shows good accuracy when the excitation and response have matching shapes, but it involves non negligible error otherwise, for example, in the case of walls of high thermal inertia.The aim of this study is to develop a simplified procedure that considerably improves the accuracy of the M&W method, particularly for excitations that noticeably depart from the sinusoidal shape, while not introducing a need for an excessive volume of data or complexity in the production of results.In the first simplified procedure discussed in the paper, a full-featured excitation is decomposed into a Fourier series and then the wall's thermal behaviour is reconstructed from the application of the M&W method to each of the N sinusoidal components. Even though this established approach can lead to the most accurate results, given a sufficiently high N, it requires the knowledge of the decrement factor and time lag associated to each component of the Fourier series, which can represent a considerable amount of data.The chief result of the research though is a novel procedure based on a parameter, γ, that weigh-averages the approximate solution obtained by considering a single term Fourier decomposition of the excitation and the solution by considering the actual excitation. The procedure is more accurate than the original M&W method and will be of interest to researchers with the means of generating values of γ for the walls which the end users of their research are interested in. It provides promising results for walls ranging from massive to negligible mass. It has been noticed that while using the same values of γ that had been optimized for the wall facing east, acceptable results are also obtained when altered external excitations are imposed, namely due to intermittency of the direct solar radiation or due to a distinct value of the external heat transfer coefficient. © 2015 Elsevier Ltd.

Vaz, D. C. "Technique to locate key points in recirculating flows, using pitot tubes." Experimental Techniques. 39 (2015): 55-68. AbstractWebsite

Abstract Saddle points (SPs), recirculation centres (RCs), and reattachment points (RPs) are examples of key points of the flow, also called critical or stationary points. Finding the location of these null velocity points is essential to the understanding of a flow pattern and may even spare measurements of the whole flow field. To that purpose, laser based techniques may render inappropriate, especially because of the difficulty in conveying seed particles to the vicinity of those points. The article details an original technique based on the measurement of pairs of profiles with Pitot tubes and finding their intersection. Two flows are taken as case-studies. These consist of jets emanating from the base of a cylindrical chamber: multiple jets arranged in a circular crown and a single central jet. While the location of SP and RP could be done in a single step, due to the symmetry of the flow, pinpointing RCs required an iterative procedure, in which the radial and longitudinal coordinates were determined alternately. The location of the RP was the least accurate. The technique has pedagogical value and will particularly interest young researchers and practicing engineers without access to elaborate and expensive techniques such as particle image velocimetry. © 2013, Society for Experimental Mechanics.

d d Ruivo, C.R.a b, Ferreira Vaz P. M. d D. "On the error of calculation of heat gains through walls by methods using constant decrement factor and time lag values." Energy and Buildings. 60 (2013): 252-261. AbstractWebsite

A transient heat transfer model was developed to numerically predict the thermal behaviour of the external walls of a room under realistic outdoor conditions. The excitation is not simply sinusoidal even though it is considered to have daily periodicity. The numerical model is based on the finite difference method and handles one-dimensional heat conduction through multilayered walls. The boundary condition at the outer surface of the wall is described with the sol-air temperature concept. The temperatures of indoor air and of other internal surfaces in the room are assumed to be equal and constant. The numerical results were used to calculate values of the decrement factor and time lag of several walls. The calculation followed two methods found in literature, in which these parameters are assumed constant, distinguished by the temperature evolution used: the sol-air or the wall's outer surface. Additionally, the inner surface temperature is used in both methods. The walls investigated range from low to high mass construction, face towards various directions and have light or dark coloured sunlit outer surfaces. The heat fluxes at the inner surface of the walls predicted by numerical modelling and estimated by the simplified methods are compared in detail to conclude on the validity of these simplified methods. As a by-product it is also possible to conclude on the dependence of the decrement factor and of the time lag on the outer surface colour and on the orientation of different types of walls. The results show that both simplified methods have poor accuracy in a significant number of cases. Also, it was found that the wall's azimuth significantly affects the time lag.© 2013 Published by Elsevier B.V.

Ruivo, C.R.a b, Ferreira Vaz P. M. c D. "Prediction of thermal load temperature difference values for the external envelope of rooms with setback and setup thermostats." Applied Thermal Engineering. 51 (2013): 980-987. AbstractWebsite

The Cooling Load Temperature Difference (CLTD) values available in the literature for using the simplified CLTD method only apply to rooms under constant indoor air temperature. Due to this limitation, the present paper extends the application of this simplified approach to cooling and heating loads estimation of rooms with daily and weekend setback and setup thermostats, and introduces the term thermal load temperature difference (TLTD). To generate the values of TLTD, a transient heat transfer model, and the corresponding numerical tool, has been developed to predict the thermal behaviour of multilayered walls and flat roofs. The sol-air temperature concept is used. The internal thermal capacity of the room is assumed negligible. The TLTD evolutions have been generated for a wall and a roof, of high mass construction, with setback and setup thermostats in winter and summer scenarios. The periods in which the room is unoccupied have been taken in due account. The TLTD evolutions allowed the estimation of: the energy transferred at the inner surface and the maximum thermal load. Among the cases studied, the relative difference found in the energy transferred through the sunlit envelope, roof or wall under summer conditions, is about 20% when the temperature control strategy is changed from A to B. © 2012 Elsevier Ltd. All rights reserved.

c Almeida, R.A.B.a, Vaz Urgueira Janeiro Borges D. C. b A. "Using ring strain sensors to measure dynamic forces in wind-tunnel testing." Sensors and Actuators, A: Physical. 185 (2012): 44-52. AbstractWebsite

The paper deals with the use of strain-gage based ring-sensors to measure the dynamic forces involved in wind-tunnel testing for research of aeroelastic instabilities. In these experiments, the model has to be supported by an elastic suspension system that allows it to move in response to the flow. The motivation for the work here reported was the assessment of the possibility of occurrence of aeroelastic instabilities in a sectional model of a bridge deck under the action of lateral wind. The model is suspended by helical springs linked to four ring-sensors and aerodynamic drag is measured by two other ring-sensors. The entire measuring system stands outside the wind-tunnel and proved to be practical, low-cost and adaptable to other sectional models. The paper describes the development process and evaluation results of the ring sensors. They feature strain gages arranged in full Wheatstone bridge circuit. The measured time series of force values were translated into lift, pitch moment and drag, as well as linear and angular displacements of the model. High quality results were obtained with the assembly, which allowed obtaining the aerodynamic parameters sought. © 2012 Elsevier B.V.

Vaz, D.C.a b, Van Kampen Van Buijtenen Kok J. F. c J. "Experimental characterization of the combustion roar in a gas turbine combustor model operating in the flame and flameless oxidation regimes." 12th International Congress on Sound and Vibration 2005, ICSV 2005. Vol. 2. 2005. 1657-1664. Abstract

Most investigations on the implementation of flameless oxidation in gas turbines have been focused on the emissions aspects while the demonstration of the combustion acoustiCS improvement has been hardly ever addressed experimentally. The present experimental study provides insight into the differences of combustion generated noise between the conventional flame and flameless oxidation regimes. Results include noise levels measured in both combustion regimes, for different air-to-fuel ratios, and show an increased noise output in flame mode as compared to flameless oxidation. The paper also explains the acoustic precautions taken to allow comparison between cases.

Moreira, A.L.N., Vaz D. "Experimental analysis of the effect of flue gas recirculation on NO x emissions in a 0.2-MW liquid-fluid furnace." Journal of the Energy Institute. 77 (2004): 59-67. AbstractWebsite

The implementation of flue gas recirculation strategies for the reduction of NOx emissions in practical liquid fuelled burners is investigated. A laboratory furnace is used and the analysis is based on detailed spatial-resolved measurements of temperature and gaseous species concentrations. It is observed that, while air staging provides major reductions in NOx emissions, with reductions up to about 30%, the effect of adding hot combustion products to the air flow is mainly due to the increase in temperature which facilitates fuel vaporization, rather than to the expected effect of oxygen dilution. However, the aerodynamics of the near burner, which is determined by the operating conditions of the burner, has a greater effect on the NO x emissions than the vitiation of the combustion air.

Vaz, D.C.a b, Van Buijtenen Borges Spliethoff J. P. a A. "On the stability range of a cylindrical combustor for operation in the flox regime." Proceedings of the ASME Turbo Expo 2004. Vol. 1. 2004. 511-516. Abstract

The implementation of flameless combustion in gas turbines is a recent research topic motivated by the potential for ultra-low NO x emissions and improvement of the combustion acoustics. This paper presents preliminary results of the implementation of flameless oxidation in a laboratory model of a cylindrical combustor (in view of stationary and micro gas turbines). Guidelines are given for the start-up of this combustion system. The experimental study then assesses, for atmospheric conditions, the combined effect of the air-to-fuel ratio, wall temperature and jet velocity on combustion stability. Global volumetric heat releases up to 16 MW/m 3, and air preheat up to 265 °C are attained. The numerical simulations give insight into the recirculating pattern of the flow and the state of mixing of reactants and products in the near-nozzle region. The turbulence-chemistry interaction is accounted for by means of the non-premixed flamelet-PDF approach (including finite rate chemistry and scalar dissipation rates up to the extinction limit).