Thermal networks considering graph theory and thermodynamics

Heat transfer in solids may be dealt with the heat equation, which is a partial differential equation, from which different analytical solutions for the study of heat transfer throughout solids and at their surfaces may be found. This implies the resolution of a distributed parameter model. On the other hand, the possibility of considering the thermal-electrical analogy is usually assumed, this being based mainly on the similarity between Ohm’s and Fourier’s laws under the assumption that the different variables used in electrical networks may be regarded as analogues to the thermal network variables. This implies the use of a lumped parameter model, which may be represented as a system of differential and algebraic equations (DAE) linked to the graphical representation of the thermal network. In this latter case the limitations of such analogy for describing heat flow should be taken into account. Therefore, it would be important to consider thermal networks independently of the thermal-electrical analogy. For this, thermal networks may be built as particular cases of directed graphs, within graph theory, since thermal networks may have physical meaning without the electrical analogy. The interpretation of a graph as a thermal network may directly use physical principles of heat and thermodynamics. This enables us to propose an alternative to the use of the electrical analogy, since electrical networks are only a particular application of graph theory consistent with electromagnetic laws which are not analogous to thermodynamic laws. Furthermore, the construction and the use of thermal networks for analysing heat transfer problems may be simplified from this perspective.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Natural ventilation in urban areas : results of the European Project URBVENT Part 1: urban environment

The application of natural ventilation is more difficult in urban than in rural environment, especially in street canyons due to reduced wind velocity, urban heat island, noise and pollution, which are considered to be important barriers to the application of natural ventilation. The wind, temperature, noise attenuation and outdoor-indoor pollution transfer were measured in a large range of variation and various types of urban configuration. The models obtained can be used in the initial stages of building design in order to assess the viability of natural ventilation in urban environment, especially in street canyons

Linear algebra solution to psychometric analysis of air-conditioning systems

International audienceThe typical air conditioning steady-state processes are graphically represented by straight or curve lines on the psychrometric chart. Neglecting the sensible heat of the moisture results in decoupling the sensible and the latent heat, that results in linear variation of the enthalpy on the psychrometric chart. The vapor saturation curve may also be linearized by using Newton's method. If the mass flow rates of the dry air are known and if the computational causality is assigned to correspond to the physical causality (i.e. if a direct modeling problem is treated), then the steady-state models of the psychrometric processes become linear algebraic equations in the vector space defined by the dry bulb temperature and the humidity ratio. Coupling these models to describe a complex HVAC (heating ventilation air-conditioning) system results in a system of linear equations that solves a direct (or psychometric analysis) problem in which the inputs of the model are a subset of the set of independent variables of the psychical process, the outputs of the model are a subset of the set of the dependent variables of the physical process, and the unknowns are the psychometric states of the moist air. The algorithm that implements this method represents a computational alternative to graphical representations and manual solutions to psychometric analysis of air-conditioning systems

Order selection of thermal models by frequency analysis of measurements for building energy efficiency estimation

International audienceExperimental identification of the dynamic models of heat transfer in walls is needed for optimal control and characterization of building energy performance. These models use the heat equation in time domain which can be put in matrix form and then, through state-space representation, transformed in a transfer function which is of infinite order. However, the model acts as a low-pass filter and needs to respond only to the frequency spectrum present in the measured inputs. Then, the order of the transfer function can be determined by using the frequency spectrum of the measured inputs and the accuracy of the sensors. The main idea is that from two models of different orders, the one with a lower order can be used in building parameter identification, when the difference between the outputs is negligible or lower than the output measurement error. A homogeneous light wall is used as an example for a detailed study and examples of homogeneous building elements with very high and very low time constants are given. The first order model is compared with a very high order model (hundreds of states) which can be considered almost continuous in space

Optimal settings of residential oil burners

International audienceResidential oil burners are capable of almost complete burning of the fuel oil, without visible smoke, when they are operated to deliver approximately 12% CO2 in the flue gases. The positions of the air damper and of the combustion nozzle are adjusted at start-up and during operation in order to maximize the combustion efficiency. In practice, one factor at a time is varied, starting with the air damper. However, this method fails to detect the interaction between air excess and nozzle position and results in non-optimal settings. Optimal designed experiments allow obtaining local regression models and statistical analyses indicate if experiment augmentation is required. The air damper and combustion nozzle settings are changed in the direction of local gradient until a second order model that contains the optimal point in its experimental region is obtained. The gain in combustion efficiency thus obtained may be up to 5% as compared with the classical approac

Natural Ventilation Potential of Urban Buildings

International audienceThe design of a building should provide the flow paths needed for natural ventilation. Therefore, the decision to apply natural ventilation should be taken early in the building design process, when little information is available for airflow estimation. To deal with this lack of data, a semi-qualitative method to assess the potential of an urban site to host a naturally ventilated building is proposed. First, natural ventilation driving forces and constraints are assessed by using comfort criteria, statistical meteorological data and user-provided information. Then, the site of interest is compared to other, well known sites using criteria related to both natural ventilation driving forces and constraints. This method compares and ranks the site within the base sites using a qualitative multicriteria analysis procedure. The result of the comparison shows if the assessed site has a higher potential for application of natural ventilation than a set of known sites