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The helical tube ground heat exchanger (GHE) surpasses all other types of vertical ground heat exchangers, such as the U-tube, double U-tube, triple U-tube, double W-tube, and double-tube type ground heat exchanger in terms of thermal performance. In the present study, the performance of several helix configurations that have an external helix outlet pipe is investigated and compared with a conventional U-tube ground heat exchanger. Another motive of this research is to find the best configuration in terms of heat transfer rate and pressure drop. Water is employed as the working fluid with a fixed flow rate of 2 L/min under laminar flow consideration and a constant inlet temperature of 300.15 K. To determine the optimal configuration among the 17 independent models, several performance parameters are considered, including heat exchange rate, outlet temperature, pressure drop, and the criterion of coefficient of performance (COP) improvement factor. Furthermore, the effects of using different materials for the tube and backfill on the performance of the GHE are evaluated. In a comparison study, it is found that a modified model with a 0.2 m pitch distance between the inlet and outlet pipes, but the upper portion of the outlet is kept straight for 8 m, provides the best thermal performance but consumes the highest pumping power. It has the highest average heat transfer among all configurations, which is 42.69% higher compared to U-tube GHE. The average heat transfer rate is increased by 6.54% for steel as a tube material compared to polyethylene. By using concrete as the backfill, there is a 5.41% improvement of average heat transfer rate compared to silica sand. The worst thermal performance is found in the U-tube ground heat exchanger. [ABSTRACT FROM AUTHOR]

In this paper, three types of horizontal ground heat exchangers (GHEs) such as U-tube, spiral, and slinky were numerically analyzed to investigate their thermal performance in cooling mode for 7 days of continuous operation with specific boundary conditions where a rectangular trench of 5 m in length, 2 m in width, and 5 m in depth served as the basis for the modeling of each heat exchanger. The pipe material was selected to be high density polyethylene for higher durability and corrosion resistance, as well as the soil and working fluid, which were clay and water. To confirm the accuracy of simulation results and reduce the computational time, a mesh independence test was performed, and simulation models were validated. There were four types of modifications, and in all of the cases, slinky GHE has better thermal performance. For instance, the heat exchange rate per unit trench length of slinky GHE was 20.72 W/m, which is higher than U-tube and spiral tube with a heat exchange rate of 9.75 and 13.62 W/m, which is of maximum 53% and 28% than U tube GHE, respectively, for the same pipe wall thickness and different material volumes of U-tube, spiral, and slinky GHEs. The pressure drop of slinky GHE is also higher (maximum 87% higher than U tube). To balance the heat exchange rate and pressure drop, the thermal performance capability (TPC) was examined. The slinky GHE has the highest thermal performance capability for all the cases. From the energy balance point, the TPC valued a maximum of 1.72 for the same material volume and different pipe wall thickness. The effectiveness was investigated to examine the heat exchange rate with different ground temperatures, and the slinky GHE showed higher effectiveness than spiral and U-tube GHEs. Finally, the thermal performance of slinky horizontal GHE was examined with different trench lengths to investigate the excavation work reduction with reduced trench length. With the decrease in trench length, the heat exchange rate and excavation work also decreased. The heat exchange rate of slinky horizontal GHEs with trench lengths of 1.5 and 1 m remained better than that of spiral and U-tube horizontal GHEs with a trench length of 3.5 m. [ABSTRACT FROM AUTHOR]

This research is focused on the experimental measurement of the thermal conductivity of CF3I and the subsequent development of empirical models, which are integral to engineering system design calculations. CF3I is identified as a potential key component in the formulation of future refrigerant mixtures, owing to its non-explosive nature, low toxicity, and minimal contribution to ozone layer depletion and global warming. Its applicability is further reinforced by its advantageous thermodynamic properties, including a low boiling point and a high critical temperature, making it a vital constituent in refrigerant mixtures. The study employs the well-established transient hot-wire method to measure the thermal conductivity of CF3I in both its liquid and vapor phases. This method is characterized by the sequential arrangement of two platinum wires which are linked in parallel to nullify any impacts from axial heat conduction. Experimental data were methodically gathered across a range of temperatures, spanning from 311 K to 374 K for the liquid phase and from 312 K to 394 K for the vapor phase, under varying pressures from 1.5 MPa to 4.0 MPa and 0.5 MPa to 3.0 MPa, respectively. The combined uncertainties associated with these measurements were meticulously calculated, amounting to 1.8% for the liquid phase and 2.0% for the vapor phase. Furthermore, the research advanced to the development of thermal conductivity models for CF3I, employing both the Extended Corresponding States method and the modified Residual Entropy Scaling method. These models effectively encapsulate the empirical findings, fitting within the established uncertainty limits with reasonable extrapolating behavior. Critically, these models are expected to play a pivotal role in predicting the thermal conductivity of refrigerant mixtures that incorporate CF3I as a component, marking a significant contribution to the field of refrigerant technology. [ABSTRACT FROM AUTHOR]

This study explores the application of zigzag-shaped, finless tubes in enhancing heat transfer performance within heat exchangers. Using three-dimensional numerical simulations, we examined the heat transfer per unit area and the volume of the pressure drop, comparing these findings with a traditional parallel tube heat exchanger. This innovative design strategy involved arranging zigzag-shaped tubes at varying distances, and the thermal transfer and frictional characteristics were tested at different air speeds. This research suggests that the introduction of zigzag heat exchangers, as opposed to traditional fin-and-tube designs, led to a significant improvement in heat transfer. This enhancement is attributed to the swirling flow created around the zigzag tubes, which increased the total heat transfer area. Furthermore, we found that the heat transfer area increased by 14.2%, 32.1%, and 63.9% for tube zigzag angles of 30°, 45°, and 60°, respectively, when compared to a parallel tube heat exchanger. Consequently, the zigzag-shaped tube heat exchanger demonstrated not only superior heat transfer, but also a reduction in frictional pressure loss. [ABSTRACT FROM AUTHOR]

Ground source cooling system (GSCS) uses a ground heat exchanger (GHE) for exchanging heat with the ground. A spiral-tube GHE is gaining interest in recent year. This study presents an experimental analysis of thermal performance of shallow spiral-tube ground heat exchanger (GHE) installed in the ground at 3 m depth in series and parallel configurations. These GHE configurations offer a compromise between the conventional vertical and horizontal GHEs. The spiral-tube GHE which is consist of spiral pipe installed in the borehole provides a better performance in application of GSCS. The thermal performances ofspiraltube GHE in series and parallel configurations were investigated under actual condition. Inlet and outlet temperatures of the both configurations were measured and periodically recorded. The average heat exchange rates of the GHEs are 122.4 W m –1 in parallel configuration and 86.2 W m –1in series configuration. Heat exchange rate of the spiral-tube GHEs in parallel configuration provides a better performance than that of in series configuration. The spiral-tube GHE in shallow depth can be applied in the GSCS.

Due to its low global warming potential (GWP) and good environmental properties, CF3I can be a suitable component of refrigerant mixtures in the field of refrigeration and air conditioning. In this work, the local condensation heat transfer characteristics of CF3I were experimentally investigated in a plate heat exchanger (PHE). The condensation heat transfer experiments were carried out under conditions of vapor qualities from 1.0 to 0.0, at saturation temperatures of 25–30 °C, mass fluxes of 20–50 kg/m2s, and heat fluxes of 10.4–13.7 kW/m2. Local heat transfer coefficients were found to vary in both the horizontal and vertical directions of the plate heat exchanger showing similar trends in all mass fluxes. In addition, the characteristics of local heat flux and wall temperature distribution as a function of distance from the inlet to the outlet of the refrigerant channel were explored in detail. The comparison of the experimental data of CF3I with that of R1234yf in the same test facility showed that the heat transfer coefficients of CF3I were comparable to R1234yf at a low vapor quality and a mass flux of 20 kg/m2s. However, R1234yf exhibited a transfer coefficient about 1.5 times higher at all vapor qualities and a mass flux of 50 kg/m2s. The newly developed correlation predicts well the experimentally obtained data for both CF3I and R1234yf within ±30%. [ABSTRACT FROM AUTHOR]