t2.5.p1

ion, equation that governs position and time distribution of temperature T (°C) is [1]: ∂T (1) = α∇ 2T ∂t where α is the thermal diffusivity (m2/s), wich is related with thermal conductivity k (W/m°C) by [2]:
where Tf is the fluid temperature, A the plate surface in contact with the fluid (m2), q the rapidity of heat transference (W) and h the convective heat transference coefficient (W/m2°C). Such coefficient depends on several factors such as velocity, temperature and kind of fluid. The Reynolds number Re determines hydromechanical similarity between the currents of the carrying heat agents, and Nusselt number Nu characterizes the intensity of convection heat interchange process [1]: ρ uD Re = (8) μ
method consists of equipment and studies simplicity. In addition, there aren’t humidity losses since not involves high temperature heating of samples. 1.2 Heat convection
EXPERIMENTAL MEASURMENTS OF THERMAL PROPERTIES FOR MEXICAN BUILDING MATERIALS TO SIMULATE THERMAL BEHAVIOR TO SAVE ENERGY
Jesus Chávez-Galán, Rafael Almanza, Neftali Rodríguez Instituto de Ingeniería, Universidad Nacional Autónoma de México Apartado Postal 20-364 México, D.F.
Nu =
hD k
(9)
θ = AU e 1 1
− m1t
(4)
The amount m is called the cooling rate in regular regime. The cooling process can be represented in T vs t graphic that would represent both stages of the cooling, nevertheless, the linear part of this curve would be the stage of regular regime, and the amount m is equal to the negative slope of this part of the curve. If local heat transference coefficient h between medium and the body tends to infinite, then m is directly proportional to thermal diffusivity of the cooled body [2]:
T = body temperature; Tm = medium temperature.
Considering a Tw temperature hot plate, Newton’s cooling law express the convection heat transference [1]:
k = cρα
(2)
where c is the specific heat (J/kg°C) and ρ the density (kg/m3). Cooling of a homogenous and isotropic body with
2
SOLAR BUILDINGS
497
arbitrary form within a medium with constant temperature and a heat transference coefficient constant respect to the time; it is described by equation 1. Therefore, temperature variation in any point follows an exponential law [2]:
q = hA (Tw − Tf )
(7)
For small t values, cooling depends on initial conditions and is a random nature process. This cooling stage is called random stage of cooling. Increasing time t, series and their terms converge quickly, except the first one that tend to zero. The process completely is determined by cooling conditions in the body’s surface, their physical properties and their dimensions. This stage is called stage of cooling in regular regime and is described by [2]:
ABSTRACT One of the main factors that determine the reliability of building’s thermal design is the values of thermal and heat transfer properties used during this process. In order to optimizing such thermal design process, there is little information available of the most utilized building materials in Mexico; hence, some measurements were carried out. We present thermal conductivity experimental results for: red brick, tepetate, adobe and concrete. Furthermore, experimental data of convective heat transfer coefficients are reported on: red brick, tepetate, adobe and concrete walls. Kondratyev methodology was used for thermal conductivity estimations. Kondratyev methodology is based on the cooling off of bodies in regular state analysis. Thermal conductivity values were: red brick kL = 0.906 W/mC, tepetate kT = 0.648 W/mC, adobe kA = 0.570 W/mC, and concrete kC = 1.918 W/mC. Red brick, tepetate, adobe and concrete test walls of 0.46x0.56 and 0.06 m thick, were manufactured, as well as a prototype of testing for mounting the walls, in order to evaluate their convective heat transfer coefficients. Measurements were carried out at the Institute of Engineering-UNAM Wind-Tunnel, for an air velocities interval of 2-10 m/s. Reported values for convective coefficients fluctuate on 16-134 W/m2°C, depending on material and position wall, as well as air velocity.
θ =
∑
i=∞ i =1
Ai U i e − m t
i
(3)
where θ = T-Tm; Ai = constant that depends of body form and initial temperature distribution; Ui = coordinates function, which describes the space distribution of temperatures; mi = constant series (m1<m2<m3<…<mi);