# Experimental and Numerical Study of Thermal Performance of an Innovative Waste Heat Recovery System
## 1 Introduction
A study by [PERSON] [14] investigated a novel high-temperature two-phase closed flat heat pipe which uses sodium as a working fluid. [PERSON] [15] performed numerical simulations on a system which proposes the introduction of heat pipes in glazed facades. There are also many applications of heat pipes in the waste heat recovery field. [PERSON] proved in his study [16] that by using oscillating heat pipes in waste heat recovery ventilation can save more than 2500 $ annually in cities with continental climatic conditions. [PERSON] [17] presented a nuclear seawater desalination system based on heat pipe technology, while [PERSON] [18] recovered waste heat from exhaust gases using a heat recovery gravity heat pipe heat exchanger. A study [19] made in 2018 presented a heat pipe heat exchanger with concentric tube heat pipes and acetone as working fluid used for low-temperature heat sources. Heat pipes were also used in designing a dual heat recovery system which recovers waste heat from residual fluids and produces simultaneously hot water and warm air [20]. A study [21] presented in 2019 analyzed the possibility of combining heat pipes with phase change materials in order to recover and store thermal energy more efficiently. [PERSON] et al. [22] designed a heat pipe heat exchanger which has the purpose to recover the heat resulted during the cooling of a ceramic kiln. The recovered heat is used to heat up an air stream which will be used in the ceramic process to dry the ceramic parts. The heat pipe-based heat exchanger was able to recover more than 863 MWh of thermal energy and also reduce the fuel consumption of the ceramic process by 110,600 \(\text{Sm}^{3}/\text{year}\) which is the equivalent of 164 tons/year of CO\({}_{2}\). In another experiment, [PERSON] et al. [23] improved the process of heat recovery in the ceramics industry using a radiative heat pipe system which was able to recover heat through radiation and natural convection in an enclosed kiln. Depending on the heater's temperature, the heat pipe can recover between 400 W and 3100 W.  
[PERSON] et al. [24] developed a heat recovery system based on a flat heat pipe heat exchanger which was tested in the laboratory and on an industrial plant. The equipment is meant to recover the radiative heat resulted during the cooling process of steel wires. The rate of heat recovery for the laboratory tests was 5 kW, while for tests made on the industrial plant the efficiency was doubled. This research presents a numerical and experimental study of thermal performance on an innovative prototype of a waste heat recovery system designed and manufactured in the laboratory of the Faculty of Civil Engineering and Building Services of Technical University \"[PERSON]\" of Iasi, Romania. The device has a modular design, with two main components, the evaporator and the condenser, and is using heat pipes for improving the heat transfer inside the device. The novelty and the originality brought by this research consists in using the axial flow for the primary and the secondary agent around the heat pipes for increasing the heat exchange surface and consequently the energy performance of the waste heat recovery system. Thus, the equipment can easily recover waste thermal energy from used water with low and medium temperatures at very low costs compared to the traditional heat exchangers, which represents an important goal in the research domain of modern heat exchangers. The recovered energy can be used for preheating or heating water for domestic use or for other heating systems without any additional pumping system or any other forms or additional energy resulting a high energy efficient innovative waste heat recovery system. The results of the experimental investigations were validated through numerical simulations.