Solar Engineering Of Thermal Processes

Melting and solidification processes of phase change material in evacuated tube solar collector with U-shaped spirally corrugated tube

Numerical evaluation of the thermal performance of a solar air heater equipped with two different types of baffles

Improvements in miniaturization and boosting the thermal performance of energy conservation systems call for innovative techniques to enhance heat transfer. Heat transfer enhancement methods have attracted a great deal of attention in the industrial sector due to their ability to provide energy savings, encourage the proper use of energy sources, and increase the economic efficiency of thermal systems. These methods are categorized into active, passive, and compound techniques. This article reviews recent passive heat transfer enhancement techniques, since they are reliable, cost-effective, and they do not require any extra power to promote the energy conversion systems’ thermal efficiency when compared to the active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluids).

/pdf/a-review-of-recent-passive-heat-transfer-enhancement-methods-128swtse.pdf

A Review of Recent Passive Heat Transfer Enhancement Methods

Generally, there are two main methods to increase the thermal performance of the heat exchanger, which include active and passive methods. Passive mode, unlike active mode, does not need external power which the heat transfer rate can be increased by utilizing changes in the flow regime or direction of the fluid path. In the present survey, numerical analysis of the effect of geometrical and operational parameters on the thermal performance of a convergent–divergent tube is done. Numerical simulations are performed using the commercial CFD code. The investigated geometrical parameters include the large and smaller diameters of the cone’s wall, the pitch of the cone and the height of the roughness. Obtained results in the first section indicate that the proposed wavy geometry leads to enhanced heat transfer in the pipe. In the second section of the study, instead of pure water, two types of water-based nanofluids, including water/Al2O3 and water/CuO, are utilized, and the obtained results are compared with pure water. Results indicate that water/Al2O3 nanofluid has better thermal performance than CuO/water and especially pure water. As a result, it can be said that for water/Al2O3 nanofluid at low Reynolds number (Re = 10,000), the case with φ = 4% has 9.29% more thermal performance than pure water which has the highest thermal performance. Also, at high Reynolds number (Re = 20,000), the case with φ = 5% has 7.15% more thermal performance than pure water. The lowest thermal performance improvement in comparison with pure water belongs to the case with φ = 2% at high Reynolds number (Re = 20,000) with about 6%. Also, for water/CuO nanofluid, the case with φ = 3% at low Reynolds number (Re = 10,000) and the case with φ = 5% at high Reynolds number (Re = 20,000) have highest and lowest thermal performance improvement in comparison with pure water with 8.86% and 6.25%, respectively.

Numerical evaluation of the effect of geometrical and operational parameters on thermal performance of nanofluid flow in convergent–divergent tube

Energy consumption is rising around the world and most of the activities in the developed countries are dependent on electric energy. Thermal energy storages are one the ways to reduce energy consumption and balance the electrical power usage during the day. With thermal energy storages, or to be precise, with ice storage systems, energy usage peak can the shifted from the afternoon or early night to low-load hours of morning or midnights. In this study, a 3D unsteady numerical analysis was done in order to examine an ice storage system equipped with serpentine tube heat exchanger. The influence of two geometrical parameters, namely, tube diameter and serpentine tube row distance were investigated. The examined range for diameter and the serpentine tube row spacing were 15–21 and 30–100 mm, respectively. The results showed that placing the serpentine tube rows more distant to each other increases the ice formation so that the ice formation rate for the highest serpentine tube row distance compared to when the serpentine tube row distance is the lowest is higher by 24.68%. On the contrary, with a larger tube diameter, the rate of ice formation was decreased so that the smallest diameter had almost 5.9% more ice formation rate in comparison with the case with the largest tube diameter.

https://link.springer.com/content/pdf/10.1007/s42452-019-1316-4.pdf

Numerical simulation of solidification process in an ice-on-coil ice storage system with serpentine tubes

Each year around 1.3 billion tons of food is wasted in the world. Some of this food waste is due to the improper performance of the cold chain, which can be controlled by the means of cold thermal energy storage devices. In this research, the charging performance of a small-scale cuboid-shaped ice container unit with two rows of serpentine tubes equipped with connecting plates has been numerically examined. The container is designed to be utilized as a cooling backup for home refrigeration units in developing countries to compensate for the heat load during the frequent power outages in these regions and to help food preservation during these times. The impacts of various dimensionless parameters regarding the flow and geometric aspects of the serpentine tubes and extended surfaces have been examined in this ice container. These parameters were the Reynolds and modified Stefan numbers of the refrigerant flow, serpentine tube pitch length/container height ratio (γ1), the serpentine tube row distance/container width ratio (γ2), the serpentine tube diameter/container diagonal length ratio (γ3), the plate area/the maximum plate area ratio (γ4), and the plate thickness/tube diameter ratio (γ5). The charging process was monitored using a dimensionless parameter, named, stored energy ratio which considers both sensible and latent energy storage progress. The results suggest that higher γ1, γ2, γ4, and γ5, and lower γ3 values lead to enhanced charging rates. It was also found that installing the full-coverage thick plates (γ4 = 1 and γ5 = 0.0081) increases the time-averaged charging rate by 18%. 

Assessment of the charging performance in a cold thermal energy storage container with two rows of serpentine tubes and extended surfaces

Frequent power outage in developing countries has created many problems for the people living in these regions, one of the most important of which is food spoilage due to the rise of refrigerator temperature. Ice storage systems are one of the promising techniques for handling this difficulty. Computational simulations are done here to influence the effects of dimensionless parameters on the charging rate of a shell and dual coil ice storage unit equipped with connecting plates as heat transfer enhancers. The ice storage unit is intended to be used as a backup cooling source for refrigerators in these regions. The studied parameters include the helical pitch length/storage height ratio (α1), the helical coil distance/storage diameter ratio (α2), the helical coil diameter/storage diameter ratio (α3), the connecting plate length/storage height ratio (α4), the connecting plate thickness/tube diameter ratio (α5), the modified Stefan number of the refrigerant flow (Ste*), and refrigerant flow Reynolds number (Re). The results suggest that the geometrical optimization of the proposed ice storage with α1, α2, and α3 parameters can improve the charging process up to 16.69%, 7.25%, and 18.84%, respectively. Also, the presence of full‐length connecting plates can enhance the charging rate by up to 12%. While the influence of the Ste* on the charging rate is considerably high (25.56%), the Re does not exhibit a noticeable effect (0.95%). Moreover, the influence of natural convection on the process was considered, however, it was found that it does not have a considerable effect on the ice formation.

Accelerating the charging process in a shell and dual coil ice storage unit equipped with connecting plates

In this paper, a numerical investigation is presented on heat transfer augmenting by using various kinds of modified multiple twisted tape inserted in solar parabolic trough collectors. The pressurized water is considered as working fluid and Reynolds number varying from 10000 to 20000. The received heat flux on the solar absorber was assumed non-uniform due to hemispheric insolation of sun. The obtained numerical results for the Nusselt number, friction factor and thermal performance are presented for each cases. Various types of twisted tape including single twisted tape, normal, perforated, center-cleared, square, and V-cut dual twisted tape are considered and analyzed. The numerical simulations are performed by a commercial CFD code, ANSYS FLUENT 18.2. The obtained results revealed that at Re=10000 and 20000, the average Nusselt numbers of case with dual v-cut twisted tapes are more than plaine tube by 19.58 and 17.44%, respectively. Moreover, thermal performance of all cases with various twisted tapes is larger than 1 which means that utilizing twisted tape with various configurations leads to more thermal performance than plain tube. The thermal performances of case with dual square-cut twisted tapes (as the best case) for Re=10000 and 20000 are more than plain tube by about 16 and 12% improvement, respectively. Furthermore, for the case with perforated dual twisted tape (as the case with lowest thermal performance), the thermal performances for Re=10000 and 20000 are more than plain tube by about 9.2 and 7% improvement, respectively.

/pdf/numerical-investigation-of-non-uniform-heat-transfer-15fpdo90al.pdf

Khashayar Pakzad

Papers

Numerical simulation of solidification process in an ice-on-coil ice storage system with serpentine tubes

Assessment of the charging performance in a cold thermal energy storage container with two rows of serpentine tubes and extended surfaces

Accelerating the charging process in a shell and dual coil ice storage unit equipped with connecting plates

Numerical investigation of non uniform heat transfer enhancement in parabolic trough solar collectors using dual modified twisted tape inserts

Numerical study of heat transfer enhancement using perforated dual twisted tape inserts in converging‐diverging tubes