대류열전달은 겨울철 온실 열손실의 중요한 원인이 되며, 일반적으로 복사열에 의한 손실보다 더 크다. 스크린의 대류열전달계수를 자연상태에서 측정한 연구가 수행된 바는 있지만 상하면의 재질이 동일하고 공극이 없는 스크린에 대해서는 적용을 할 수 없는 방법이다. 이러한 재질의 스크린은 한국에서 많이 사용되고 있으나 대류열전달 특성을 파악하는데 많은 어려움이 있는 실정이다. 본 연구에서는 공극이 없는 3가지 종류의 스크린에 대해 대류열전달계수를 구하였으며, 계수를 산정하기 위하여 복사열수지 이론에 근거하여 산정방법을 개발하였다. 실험장치에 스크린을 설치하고 일사량, 장파복사량, 대기온도, 스크린 및 흑색천의 표면온도, 풍속 등을 측정하였다. 스크린의 표면온도와 주변온도의 차이에 따른 대류열전달계수를 산정하였다. 풍속이 거의 없는 상태에서 온도의 차이가 증가함에 따라 계수는 감소하는 것으로 나타났다.
대류열전달은 겨울철 온실 열손실의 중요한 원인이 되며, 일반적으로 복사열에 의한 손실보다 더 크다. 스크린의 대류열전달계수를 자연상태에서 측정한 연구가 수행된 바는 있지만 상하면의 재질이 동일하고 공극이 없는 스크린에 대해서는 적용을 할 수 없는 방법이다. 이러한 재질의 스크린은 한국에서 많이 사용되고 있으나 대류열전달 특성을 파악하는데 많은 어려움이 있는 실정이다. 본 연구에서는 공극이 없는 3가지 종류의 스크린에 대해 대류열전달계수를 구하였으며, 계수를 산정하기 위하여 복사열수지 이론에 근거하여 산정방법을 개발하였다. 실험장치에 스크린을 설치하고 일사량, 장파복사량, 대기온도, 스크린 및 흑색천의 표면온도, 풍속 등을 측정하였다. 스크린의 표면온도와 주변온도의 차이에 따른 대류열전달계수를 산정하였다. 풍속이 거의 없는 상태에서 온도의 차이가 증가함에 따라 계수는 감소하는 것으로 나타났다.
Convective heat transfer is the main component of greenhouse energy loss because the energy loss by this mechanism is greater than those of the other two components (radiative and conductive). Previous studies have examined the convective heat transfer coefficients under natural conditions, but they...
Convective heat transfer is the main component of greenhouse energy loss because the energy loss by this mechanism is greater than those of the other two components (radiative and conductive). Previous studies have examined the convective heat transfer coefficients under natural conditions, but they are not applicable to symmetric thermal screens with zero porosity, and such screens are largely produced and used in Korea. However, the properties of these materials have not been reported in the literature, which causes selectivity issues for users. Therefore, in this study, three screens having similar color and zero porosity were selected, and a mathematical procedure based on radiation balance equations was developed to determine their convective heat transfer coefficients. To conduct the experiment, a hollow wooden structure was built and the thermal screen was tacked over this frame; the theoretical model was applied underneath and over the screen. Input parameters included three components: 1) solar and thermal fluxes; 2) temperature of the screen, black cloth, and ambient air; and 3) wind velocity. The convective heat transfer coefficients were determined as functions of the air-screen temperature difference under open-air environmental conditions. It was observed from the outcomes that the heat transfer coefficients decreased with the increase of the air-screen temperature difference provided that the wind velocity was nearly zero.
Convective heat transfer is the main component of greenhouse energy loss because the energy loss by this mechanism is greater than those of the other two components (radiative and conductive). Previous studies have examined the convective heat transfer coefficients under natural conditions, but they are not applicable to symmetric thermal screens with zero porosity, and such screens are largely produced and used in Korea. However, the properties of these materials have not been reported in the literature, which causes selectivity issues for users. Therefore, in this study, three screens having similar color and zero porosity were selected, and a mathematical procedure based on radiation balance equations was developed to determine their convective heat transfer coefficients. To conduct the experiment, a hollow wooden structure was built and the thermal screen was tacked over this frame; the theoretical model was applied underneath and over the screen. Input parameters included three components: 1) solar and thermal fluxes; 2) temperature of the screen, black cloth, and ambient air; and 3) wind velocity. The convective heat transfer coefficients were determined as functions of the air-screen temperature difference under open-air environmental conditions. It was observed from the outcomes that the heat transfer coefficients decreased with the increase of the air-screen temperature difference provided that the wind velocity was nearly zero.
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문제 정의
In this study we describe a new radiation balance method for heterogeneous (made of more than one material) symmetric screens. The first purpose of this study is to determine the heat flux and convective heat transfer coefficient of greenhouse screens.
In this study we describe a new radiation balance method for heterogeneous (made of more than one material) symmetric screens. The first purpose of this study is to determine the heat flux and convective heat transfer coefficient of greenhouse screens. The second is to draw a relationship between convective heat transfer coefficients, air velocity, and air–screen temperature differences.
가설 설정
10. The other four factors (g, L, β & ΔT) have direct relationship with ratio but have minor effect on the values.
The effect of low wind speed can be seen in Fig. 9. The ratio of GrㆍRe-2 shows higher values due to the low wind speed, which is directly related to purely free convection. More than 98% of values of this ratio for all tested samples were greater than 1.
제안 방법
A mathematical model was presented to experimentally calculate the convective heat transfer coefficients of energy-saving screens. The procedure was based on radiation balance equations, and it was applied to a wooden frame on which horizontal screens were tacked.
The second is to draw a relationship between convective heat transfer coefficients, air velocity, and air–screen temperature differences. The final purpose is to separate natural, forced, and mixed convection by using the GrㆍRe-2 ratio.
A mathematical model was presented to experimentally calculate the convective heat transfer coefficients of energy-saving screens. The procedure was based on radiation balance equations, and it was applied to a wooden frame on which horizontal screens were tacked. Basic parameters required to calculate the coefficient value were radiative properties and air–screen temperature difference.
, 2015). These studies dealt with homogenous (screens made of one material only) and symmetric (screens with identical sides) materials. With the development of new greenhouse screens (heterogeneous and asymmetric), their application to new cases is limited.
This research was carried out on a building roof in order to get clear sky radiation. A hollow wooden frame was built for the investigation.
대상 데이터
This research was carried out on a building roof in order to get clear sky radiation. A hollow wooden frame was built for the investigation. The bottom part of the frame was covered with black cloth with known radiometric properties (τb =0, ρb =0.
Basic parameters required to calculate the coefficient value were radiative properties and air–screen temperature difference. Equipments used for this study were solar and thermal radiation sensors (Net Radiometers, Pyrgeometer, and Pyranometer), temperature sensors (Thermocouple and Hobo Pro v2 U23-002) and anemometer. The convection heat transfer was always purely free for all tested samples, the Reynolds numbers showed that the airflow was in the laminar regime, and the wind velocity in the local area was less than 1 mㆍs-1.
성능/효과
The convective heat exchanges between the upper or lower side of the thermal screen and air were calculated, and they are demonstrated in Fig. 7. LD-15 and LD-13 transferred high amounts of heat flux to the air during the daytime because both had high absorption in the shortwave region, which subsequently raised its temperature as compared to ambient air temperature during the daytime. Cloudiness condition can affect the value of heat flux, as the flux is purely based upon solar, sky and materials radiations.
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