Mohammad Rahiminejad

Mohammad Rahiminejad is a third-year Ph.D. student in the doctoral program of Mechanics at EPFL. His main research centers on building physics and examines how the thermo-hydrodynamic behavior of airflow in a ventilated cavity behind traditional and modern (BIPV) external claddings impacts the thermal performance of the entire building envelope. To tackle this topic, he takes a multidisciplinary approach that encompasses numerical analyses, CFD simulations, and experimental measurements. Before joining EPFL, he worked as a project manager assistant in Shiraz, Iran, on quite a few industrial projects including the design of the ventilation systems of Tehran-Shomal Freeway, design of the air conditioning systems of Shiraz subway, and design of the air exhaust and jet fan systems of the Shiraz longest underpass. He holds a master’s degree in Mechanical Engineering from the Sharif University of Technology, Iran, where he investigated water purification through monolayer graphene membranes using Molecular Dynamics (MD) simulation. In his bachelor's degree in Shiraz University, he successfully achieved publishing a journal paper in the field of biomechanics that addressed the distribution of the pressure and velocity fields in the human upper airway during sneezing using CFD simulations.

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The EPFL-UNIL campus is today at the heart of an intense institutional reflection on its future. The ENAC week aims to carry out specific operational proposals for the campus starting from a reflection on public space, climate change and and built environment.

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Building envelope

A building envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer. Discussi

Brick

A brick is a type of construction material used to build walls, pavements and other elements in masonry construction. Properly, the term brick denotes a unit primarily composed of clay, but is now

Building

A building or edifice is an enclosed structure with a roof and walls, usually standing permanently in one place, such as a house or factory. Buildings

Experimental study of the hydrodynamic and thermal performances of ventilated wall structures

Dolaana Khovalyg, Mohammad Rahiminejad

The presence of a ventilated air gap behind the external cladding in a building envelope is known to have a tangible contribution to the overall performance of the wall assembly. In the present study, the hydrodynamic and thermal performances of ventilated wall structures in a full-scale test facility are experimentally investigated using data collected during a full-year measuring campaign. In particular, the impact of varying outdoor conditions, façade orientation, and thermal inertia of the walls on the airspeed in the cavity, temperature distribution on the surfaces, heat flux through the interior space, and heat flow in the air gap are evaluated. Additionally, the discussion on transient responses of the wall structure, driving forces applied to the airflow, the overall heat gain/loss through the interior surface of the wall, and heat recovery potential from the cavity are provided. Moreover, variation of the thermal resistance caused by the ventilated air-space is analyzed. The results reveal the importance of monitoring the outdoor microclimate conditions adjacent to the wall of interest to examine the performance of the wall structure properly. It is observed that the airflow characteristics in the cavity are better correlated with outdoor temperature than other weather parameters. It is shown that the thermal resistance of the ventilated cavity dynamically varies and is generally higher in the summertime compared to the wintertime. The results highlight the potential of harvested heat flow from the ventilated cavity reaching 158 kWh/month, which could be used as an additional energy source for buildings. The optimized air change rate in the air-space obtained in this research to simultaneously provide high cavity thermal resistance and high heat recovery from the air gap shows the necessity for considering a fan system to supply specific airflow in the cavity.

2023

Heat recovery potential from ventilated passive and active facades

Dolaana Khovalyg, Mohammad Rahiminejad

A ventilated air-space behind external claddings can potentially affect the thermal performance of the entire building structure. In particular, in the Building Integrated Photovoltaic (BIPV) facades, ventilated cavities are typically present between the PV panels and the walls of the building. The airflow in the cavity can remove the generated heat behind the active external cladding, which could be eventually used as an additional source for heat recovery. In this study, the heat recovery from a ventilated air-space behind passive (wood) and active (BIPV) facades are investigated using transient simulations. The numerical model used in this study is validated against experimental measurements carried out in a building prototype located in the Smart Living Lab in Fribourg (Switzerland). The original façade is made of wooden cladding that is separated from the wall core incorporating a ventilated cavity. To study the impact of façade type on the results, the external cladding is virtually replaced with typical polycrystalline PV panels. The analyses are performed for representative days in the winter and summer of 2021 using recorded weather data on the test building. The results are examined in terms of the temperature distribution of the layers in the wall assembly, heat flux through the indoor space, airspeed in the cavity, and heat flow in the air gap. The potentials for heat recovery per day of interest are also calculated and compared. It was shown that the heat recovery from the cavity behind the BIPV façade could become equal to 5341 kWh on a representative summer day, which is considerably higher compared to the value obtained for a passive cladding. The results highlight the potential for harvesting heat from the ventilated air gaps behind passive and active facades. The outcome of this study highlights the need for the integrated vision for energy-savings at the building scale. CLIMA 2022 conference, 2022: CLIMA 2022 The 14th REHVA HVAC World Congress

TU Delft OPEN2022

Impact of the ventilated air-space behind traditional (passive) and BIPV (active) façades on the thermo-hydrodynamic performance of the building wall structures

Mohammad Rahiminejad

For many decades, ventilated cavities in wall assemblies of buildings have been essential for creating moisture-resilient constructions by allowing airflow within the air gap to promote drying. In addition to that, the airflow in the cavity can contribute to the overall performance of the wall structure. Similar to the wall assemblies with traditional claddings, photovoltaic (PV) panels in the Building-integrated photovoltaic (BIPV) façades can be separated from the building envelope by an air gap that is created for ventilation purposes. In this study, the impact of the thermo-hydrodynamic behavior of the airflow in the naturally ventilated cavity behind traditional (passive) and BIPV (active) façades on the performance of the entire wall assembly is numerically and experimentally investigated. Due to the complexity of the problem caused by the contribution of several parameters, as a first step, a comprehensive literature review is performed to specify the effective factors on the air change rate in the ventilated cavity behind common wall assemblies. As the second step, the steady-state analysis is carried out within the framework of a research project ASHRAE 1759-RP funded by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), in which the plausible definitions of the thermal resistance of a vertical ventilated air-space behind external cladding systems are defined using theoretical and applied formulations. Moreover, based on the uncertainty analysis of measurements, the modifications of the relevant standardized method (ASTM C1363-19) to account for the airflow effects on the thermal performance of the wall assembly are proposed. In the third step, the problem is further explored in the transient conditions by developing a 2D numerical in-house code validated against experimental measurements. Accordingly, the performance of wall structures with different thermal inertia having passive (traditional) and active (BIPV) external claddings are numerically investigated. In the last step of this thesis, the problem is experimentally discovered, and the thermo-hydrodynamic performance of ventilated wall structures of the test facility is investigated using data collected during a full-year measuring campaign. The results of this study show that the thermo-hydrodynamic behavior of the airflow behind external claddings is a complex phenomenon. The results of the steady-state analysis reveal that, under some conditions, the air-space could be able to resist the heat flux passing the wall assembly even more than the external cladding. Moreover, the uncertainty analysis sheds light on the fact that accurate instruments are needed to reduce the uncertainty of measuring the thermal resistance of the cavity. The results of the transient numerical analysis show that the overall performance of a ventilated wall structure dynamically changes depending on the diurnal variation of the outdoor/indoor conditions. Moreover, it is shown that replacing the passive cladding with an active façade could strongly affect the hydrodynamic and thermal performances of the entire wall assembly. The results of the experimental measurements pointed out that the airflow in the ventilated cavity has a major impact on the yearly performance of a wall structure; therefore, further investigations are required in future studies to evaluate the possibility of controlling the air change rate in the cavity using a fan-assisted system.

EPFL2022