ارايه‌ي رابطه‌ي كاربردي جديد براي برآورد ميزان چگالش در سيستم‌هاي سرمايش تابشي

DEVELOPING A NEW APPLICABLE CORRELATION TO ESTIMATE CONDENSATION RATE ON RADIANT CEILING COOLING PANELS

سيستم سرمايش تابشي سقفي به‌دليل مصرف انرژي پايين و فراهم آوردن شرايط آسايش حرارتي مطلوب امروزه مورد توجه ويژه‌يي قرار گرفته است. پديده‌ي چگالش بخار آب موجود در هوا روي پانل‌هاي سرمايش تابشي سقفي همواره به‌عنوان محدود‌كننده ترين عامل توسعه‌ي اين سيستم‌ها محسوب شده است. در اين تحقيق پانل‌هاي سرمايش سقفي در شرايطي كه احتمال رخداد چگالش زياد است مورد بررسي قرار گرفته‌اند و نرخ چگالش و عوامل موثر بر آن به‌ازاي طيف وسيعي از شرايط محيطي توسط مدل‌سازي عددي جريان هواي مرطوب محاسبه شده است. پارامترهاي سرعت، دماي شبنم هوا و دماي پانل سقفي به‌عنوان مهم‌ترين پارامتر هاي موثر بر نرخ چگالش شناسايي شده‌اند و يك رابطه‌ي كاربردي ساده براي محاسبه‌ي نرخ دقيق چگالش برحسب پارامترهاي موثر ارايه ‌شده است.

Radiant ceiling cooling (RCC) systems have gradually attracted the attention of HVAC engineers because of the possibility of providing good thermal comfort conditions without the need for large energy consumption. It should be mentioned that the RCC systems are constructed from flat ceiling-mounted radiant panels with a high absorption coefficient. In these systems, the chilled water is passed through the cooper tubes and causes the panel temperature to decrease. Although the RCC systems have many advantages, like high thermal efficiency, by providing good thermal comfort conditions, the performance of these systems can be limited by the condensation of water vapor on the chilled ceiling. Therefore, radiant ceiling cooling systems may be impracticable in hot and humid climates. The main aim of this study is to analysis the performance of the RCC systems at the vicinity of dew point temperature and under conditions wherein the risk of condensation is very high. For this purpose, the average value of condensation rate in a wide range of temperature and relative humidity has been numerically calculated by developing a CFD code for simulating heat and mass transfer over the chilled panel. The mentioned CFD code can solve four main governing equations; continuity, momentum, energy and concentration equations, by the finite-difference technique. Results indicate that air speed, dew point temperature and surface temperature of the chilled panel are the most effective parameters to affect the condensation rate on the chilled panel. After finding these important parameters, a simple, non-differential correlation has been proposed by linear regression on the basis of the mentioned effective parameters. The correlation has been defined using the analogy between the heat and mass transfer phenomena. The correlation has been verified in two different ways. First, by comparing the dimensionless velocity profile inside the boundary layer with the similar analytical solution, and second, by comparing the results of the correlation with a mass transfer correlation proposed by Robert E. Treybal, in 1981. Both validations show very good agreement between the mentioned comparative results. Therefore, the mentioned simple correlation can be extensively utilized for designing a radiant ceiling cooling system under various climactic conditions.