MEASUREMENT OF DISTRIBUTION OF TEMPERATURE AND MOISTURE CONTENT WITHIN WOODEN PLATE UNDER STEADY HEATING AND NUMERICAL REPRODUCTION WITH HEAT AND WATER TRANSFER ANALYSIS

Abstract

 Distributions of moisture content and temperature within cedar specimens under steady heating were measured with thermocouples and moisture sensors developed in ref.¹⁰⁾, and compared with heat and water transfer analysis. Increasing of moisture content caused by heat ant water transfer was measured in experiments, and enable to be represented by analysis.

 Moisture content as well as temperature affects fire resistant performance of wooden member during fire. In particular, charring rate become slow because of heat capacity of water, and mechanical properties decrease significantly even under 100℃ at high moisture content. In most literatures, fire resistant performance had been simply predicted with only temperature because of lack of a measurement system of local moisture content.

 Cedar specimens whose thickness was 45mm located thermocouples and developed moisture content sensors¹⁰⁾ were heated from flat grain by cone calorie meter. Located moisture content measurement systems calculated moisture content with electric resistance between two electrodes for each measurement point in specimens, and measurement range is most effective at 10~30 % moisture content. Each three Specimens were heated 60 minutes by 4.5 kW/m², and 30 minutes by 20 kW/m². Local moisture content increased gradually to 27.8% in maximum even temperature was under 100℃ by 4.5 kW/m² heating, increased temporarily to over 30% in maximum by 20 kW/m² heating. Increasing of moisture content was caused under 100℃, thereby attributed to water evaporation in area at about 100℃, transfer, and re-condensation. Time variations of moisture content was differed from each specimen even at the same heat strength.

 Temperature, moisture content, total pressure and vapor pressure were coupled and calculated by Crank-Nicolson method in the model for heat and water transfer. Changes of moisture content is caused by adsorption/desorption rate defined explicitly. Specifically, water evaporation in higher temperature area, vapor transfer, increasing of vapor pressure in lower temperature area, increasing of relative humidity, increasing of equilibrium moisture content and increasing of adsorption rate cause increasing of moisture content.

 The agreement for temperature between experiments and calculation results was good; however, improvement of accuracy of the moisture content measurement system and investigation of influence of grains of wood were needed because quantitively comparison was difficult by difference of time variation of moisture content at the same heat strength. Closer to unheated surface, difference between experiments and calculation results of moisture content became larger. In addition, modeling of the process of char oxidation and mass decrease and improvement of adsorption- isotherm will be necessary because temperature increasing was a bit slower and the maximum value of moisture content in experiments was bigger than maximum values of adsorption-isotherm.

 In the future, two-dimensional analysis and prediction for distribution of temperature and moisture content within a full-scale member exposed to unsteady heating is required. Furthermore, prediction and evaluation of influences of heat and water transfer for mechanical properties is important.