Assessing the impactof passive cooling on thermal comfort inlig house using CFD

Abstract

Keywords: Thermal Comfort; Passive Cooling; Green Roof and Walls

Introduction

On account of modernized ambience, readily available comfort enhancing devices and lack of space, most housing layouts and constructions neglect the possibility of providing comfortable environment naturally. Dependency on external cooling sources has been increasing day by day resulting in increase in energy consumption. This leads to non-conducive living environment and environmental damage. Natural enhancement of thermal comfort, on the other hand, is environment friendly, cost-effective and easily maintainable. Investigation of the thermal performance of buildings help in ascertaining the thermal comfort or lack of it in modern buildings, and the studies allow for avenues of incorporating thermal comfort in a cost-effective manner. Raipur (21.25ºN, 81.63ºE) city is the capital of Chhattisgarh, India, as it experienced the maximum temperature of about 46 ºC at peak summer, a thermal performance analysis of houses for this region can help in assessing the level of thermal comfort. Based on ASHRAE, the National Building Code of India advocates the use of two indoor temperature ranges for thermal comfort; 23 °C to 26 °C for summer and 21 °C to 23 °C for winter. It has been observed that existing LIG houses hardly provide thermal comfort in Raipur region of Chhattisgarh. The objective of the present work is to investigate and enhance the thermal comfort of LIG house by passive cooling techniques. Any treatment on the building which will reduce the indoor temperature of the building by reducing heat gain to the building is known as passive cooling techniques. Passive cooling systems used in buildings are naturally ventilated buildings, nocturnal ventilative cooling, direct evaporative cooling, indirect evaporative cooling, cooling the soil and green roof/wall. Day time ventilation is used to reduce the indoor temperature; this technique is known as naturally ventilated buildings. Use of ventilation at night time to reduce the indoor temperature is known as nocturnal ventilative cooling. Direct evaporative cooling techniques and indirect evaporative cooling, reduces the indoor temperature by water evaporation direct (desert cooler) and indirect (pond water) method, respectively. Cooling the soil technique uses cool soil as a heat sink to reduce the indoor temperature, and green roof/wall uses vegetation on walls/roof to reduce the indoor temperature. The green roof consists of water proofing layer, drainage, soil and vegetation. Mathematical model for thermal performance analysis of building by using MATLAB /SIMULINK was developed [1]. Result also shows that high thermal mass building reduces room air temperature of the building. [2] Reported that ventilation improves the physiological comfort of the occupant by creating airflow it is also concluded that if the ambient temperature is higher than the indoor temperature it will increase the room temperature of the building. [3] Presented results from field surveys and simulates the effects of window opening behaviour of office buildings on thermal comfort and energy. When the ambient temperature is lower than indoor temperature, opening a This paper was recommended for publication in revised form by Regional Editor Sandip Kale 1,2,3 National Institute of Technology Raipur, 492010, Chhattisgarh, India *E-mail address: sbhowmick.mech@nitrr.ac.in Orcid id: 0000-0001-8536-5524, 0000-0001-5435-630X, 0000-0001-9799-8724 Manuscript Received 12 January 2018, Accepted 24 March 2018

Journal of Thermal Engineering, Technical Note, Vol. 5, No. 5, pp. 414-421, October, 2019

window reduces the indoor temperature of the building. Ventilation during the evening is important in hot, humid climates to reduce the discomfort level, because it increases the air speed and enhances sweat evaporation [4]. Simulated the cool roof in Design builder tool, to improve indoor thermal conditions [5]. Results in the study show that a vault roof with high reflectivity coating reduces 53% in discomfort hours during the summer season as compared to flat roof building. Different passive cooling techniques inside test structures were investigated [6] and concluded that pieces of white glazed tiles fixed over the roof is the best method of cooling of buildings for better thermal comfort conditioning in arid regions. Effect of the window opening in LIG house, at peak hour in summer has been reported. It is concluded that opening of windows at summer increased the indoor temperature. ANSYS fluent are used in the present study to assess the indoor air temperature of the house. Incorporating green wall, green roof and shading devices are some of the passive cooling techniques to reduce the heat input and enhance temperature of the building are discussed in present work. Different cases are considered in the present study are, the effect of windows opening and size, green roof and green walls and shaded wall (External projection on the wall) on the indoor temperature of LIG house for May 20, 2016.This information will be helpful for the designer to design a house which will provide thermal comfort with less dependency on cooling devices.

Basic Assumptions, Initial And Boundary Conditions

To determine indoor temperature distribution in LIG house different parameters with different boundary conditions and setting were studied. LIG house was simulated with closed windows, opened windows, green roof and green walls and shaded wall. Intermediate temperature between vegetation soil and concrete roof has been calculated using the conduction heat transfer. It is found that green roof reduces the temperature by 5.43 °C therefore; simulation has been carried out with the concrete roof and reduced temperature. The similar method has opted for green walls. The following assumptions were considered for model simulation:

4. Convective heat transfer coefficients are constant.

The governing Navier-Stokes equations and radiation model were solved in the three dimensional, parallel, finite-volume solver ANSYS FLUENT v17.1. The cases were solved as stationary, on May 20, 2016. A widely used, the standard k-ε model was used for closing the set of governing equations. Convective heat transfer coefficient for outer wall and the inner wall is taken as 22.7 W/m2.K and 8.3 W/m2.K [8]. Total solar radiation incident on horizontal and vertical surfaces, which is calculated as follows [9]:

In Eq. 15, Id is direct radiation, Idiff is diffused solar radiation, β is wall inclination angle from horizontal, ρ is ground reflectance (usually 0.2 for normal condition) and θ is the solar incident angle.

 sin  sin  cos   sin  cos  sin  cos   cos  cos  cos  cos     cos  sin  sin  cos  cos   cos  sin  sin  sin  

Journal of Thermal Engineering, Technical Note, Vol. 5, No. 5, pp. 414-421, October, 2019 Here, n is the day of year, φ is the latitude, γ is Surface azimuth angle (−1800 ≤ 𝛾 ≤ 1800 ) and ω is hour angle. Solar radiations on walls have been calculated from the Eq. (1), and are shown in table 1.

Methodology

Based on the building plan, the 3D model has been created in SOLIDWORKS (Figure 1); the model is imported in ANSYS fluent for thermal performance analysis. Parametric simulation with the ANSYS fluent was used for analysing the indoor comfort within a LIG house for the climatic condition of Raipur. Indoor temperature of house, with closed windows has been measured using Testo 605i at the height of 3 feet and 7 feet. Results show good agreement with average deviation of 3.7 %.

Results

The simulation results have been validated with the onsite data collected using temperature sensors on 20 th May 2016 for house with open window (Room 2) and reported in Table 3. Table 3. Validation of indoor temperature

Conclusion

Through parametric simulation, the effect of window sizes and passive cooling techniques on indoor temperature has been studied. The study shows that the sizes of the window on the building do not affect indoor temperature significantly in summer at early morning. In summer during night time, when the outside temperature is lower than the indoor temperature, it reduces the room temperature but is insufficient to provide thermal comfort. It is also concluded that providing green roof along with green walls enhance the indoor temperature of the house by approximately 4°C, it gives better results as compared to other passive techniques.

Nomenclature

C Specific heat (J/kg.K) ho Convective heat transfer coefficient at outer wall k Thermal conductivity (W/m.K) T Temperature (K) Tamb Ambient temperature (K) U Overall heat transfer coefficient (W/m2.K) u,v,w Velocity (m/s) ρ Density (kg/m3) Ambient amb

Share and Cite