On the comparison between compound louvered-vortex generator fins and X-shaped louvered fins

A recent evolution in heat exchanger design is the use of compound designs. One of the designs under study is a combination between a louvered fin and vortex generators. Several possible placements of the vortex generators are studied. These compound designs are compared with the X-shaped louvered fin, which maximizes the louvered area. It is shown that the X-shaped louvered fin exhibits the same heat transfer enhancement mechanism as the compound design, with respect to the rectangular louvered fin. The X-shaped louvered fin outperforms all of the compound designs

Optimisation of compound louvred fin and vortex generator heat exchangers

Compact fin and tube heat exchangers are extensively used in heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications. Specialised fin geometries such as the louvred fin allow the construction of heat exchangers with less material use and low fan powers. Due to the scarcity of resources, many investigations of the fin geometry have been conducted to improve heat exchanger performance even further. These investigations revealed that the use of vortices was an interesting approach, resulting in the vortex generator fin design. However, there are many degrees of freedom regarding the geometry of these vortex generators. In order to obtain a better performance than the fin geometries which are already commonly used, optimisation of the vortex generator geometry is required. Very recent work has showed that the combination of louvres with vortex generators has significant potential. The optimal geometry of the vortex generator geometry and positioning in these types of fins is an open question, which is dealth with in this work. The first issue which needed to be resolved was how to perform optimisation of fin geometries. Every fin geometry has different pressure drop and heat transfer characteristics. In general, there is a trade-off between these two characteristics, increases in the heat transfer tend to result in increases in the pressure drop. Many different performance evaluation criteria are available in the open literature. When one of these criteria is used as a goal function in an optimisation routine, it is important to be aware of the physical significance of these criteria to understand just in what way the obtained fin geometry is optimal. Several recent studies have used criteria which have no clear physical meaning to perform the optimisation. By modifying the existing approaches, a new method was developed which has a clear physical interpretation. This method was used to optimise the louvred fin and round tube geometry for low velocity HVAC applications, resulting in a geometry which required a lower fan power for the same volume or a lower volume for the same fan power and mass flow rate. Geometries with larger louvre angles were found to outperform geometries with lower louvre angles. The performance evaluation criteria use specific assumptions to result in simple analytic equations, allowing them to be used for quick comparisons between vastly different fin geometries. However, their simplicity is of no particular value if they are used to optimise geometries using computational fluid dynamics (CFD) simulations. In this case, accuracy is far more important than the simplicity of the equation. The impact of relaxing these assumptions on the predicted heat exchanger performance was therefore investigated. One such assumption was that two critical heat exchanger parameters, the Colburn j-factor and the friction factor were independent of the heat exchanger length. This assumption resulted in the prediction that the heat exchanger volume could be reduced by increasing the frontal velocity and increasing the heat exchanger length. If this assumption was abandoned and the effect of the length on these parameters was taken into account, it became apparent that this prediction was erroneous. It was shown that if the heat exchanger length was constrained and the mass flow was varied, the optimal geometry depended on the number of transfer units of the heat exchanger. For a large number of transfer units, the plain fin geometry outperformed the enhanced fin designs, whereas for a low number of transfer units, the enhanced fin designs performed better than the plain fin. In contrast, if the mass flow rate was constrained and the length was allowed to vary, the enhanced fin geometries always outperformed the plain fin. Another option was to constrain both the mass flow rate and the heat exchanger length and to allow the hydraulic diameter to vary. This resulted in louvre geometries with larger louvre angles requiring larger hydraulic diameters and therefore also a larger volume for the same fan power. Clearly the optimal fin geometry depends on the constraints, which shows the importance of having a clear physical interpretation to determine when a certain geometry is optimal. Due to the relatively large number of geometrical parameters, the number of different geometries which need to be evaluated is quite large. Since the simulations are computationally expensive, it is not possible to investigate the entire behaviour of every parameter of interest. The design of experiments methodology allows obtaining virtually the same information with a vastly reduced amount of data under certain conditions. The Taguchi method in particular is quite popular due to its impressive reduction in required data and large flexibility. However, an essential requirement for this method is that there are no interactions between different parameters. Whether this is valid or not was not investigated for the compound louvres and vortex generator fin. It is revealed in this work that interaction effects between the louvre angle and the vortex generator parameters are very important. An interaction between four different parameters was generated as a result of the interaction between the position of the trailing edge of the vortex generator and the tube wake. A performance screening of the compound fin showed that it was possible to do better than the current state-of-the art X-shaped louvred fin, reducing the required heat exchanger volume by 7%. The way the vortex generator geometry is parametrised is essential to obtain good performance. In order to understand why this compound geometry performed better than the X-shaped louvred fin, it was necessary to further investigate the behaviour of the fin. A new method was developed to obtain the fin efficiency from CFD simulations as a post-processing step. By applying this method to the geometries of interest, it was revealed that the main reason for the difference in performance between the X-shaped louvred fin and the compound design was due to the superior fin efficiency of the compound design. This showed that the optimal fin geometry depended on whether fin efficiency effects were taken into account for the evaluation or not. Furthermore, the generation of the vortex was not more effective than the louvre geometry which was already used. By studying the flow field of the different geometries in detail, it was revealed that vortices were already present in the regular louvred fin. The addition of an additional vortex by means of the vorte

Local heat flux measurement technique for internal combustion engines

The heat transfer from the combustion gases to the cylinder wall affects the efficiency, emissions and power output of an internal combustion engine. Measuring the heat transfer requires a heat flux sensor inside the combustion chamber that has a short response time and is able to withstand the harsh conditions during combustion. In this work, a suitable sensor is introduced and the measured wall temperature, heat flux and convection coefficient are compared to those measured with a commercial sensor. It was found that both sensors measure the same convection coefficient, but a different wall temperature and heat flux. This is because the presence of the sensor in the combustion chamber wall affects these quantities. A method is proposed to cancel this effect and calculate the actual heat flux through the cylinder wall

Combined conduction and natural convection cooling of offshore power cables in porous sea soil

The power that can be carried by offshore power cables is often restricted by the temperature limit of the materials inside the cable. It is therefore essential to predict the heat transfer behavior of the dissipated power from the cable to the environment. Offshore cables are buried in the seabed, which is a porous structure of sea soil saturated with water. Both conduction of heat through the soil, as well as natural convection due to the flow of water through the porous soil, are possible ways of heat transfer. Most cases are best described as a combination of these heat transfer effects. In this paper, a numerical model is made to predict the heat transfer from the cable to the environment by modeling the surrounding soil as a porous medium. The influence of soil parameters such as conductivity, heat capacity and permeability, as well as geometrical parameters, such as burial depth and cable diameter, are tested. An analytical expression, which can estimate the heat transfer rate for conduction dominated heat flows, is used. For convection dominated heat flows, a correlation in function of the Darcy-modified Rayleigh number is used. For heat flows which are a combination of conduction and convection effects, an algebraic summation of the thermal conductance due to convection and conduction is found not to give adequate agreement with the simulations. It is shown that an asymptotic expansion of the limiting equations for conductive and convective heat transfer rate can be used to determine the total heat flow effectively. Several soil samples in the North Sea are analyzed, and the thermal properties are used as inputs for the model. These calculations show that conduction is the main heat transfer effect and that convection has a limited effect on the heat transfer

Determining heat losses in a reheat furnace : a case study

In a previous study of an actual reheat furnace with a capacity of 45 ton/h, it was found that a staggering 20% of the input energy was unaccounted for in the heat balance. It was hypothesized that this missing energy was lost to the environment as heat losses. In this work, the main losses are identified and quantified. First, the heat transfer through the wall of the furnace was determined. For this, an extensive measurement campaign was performed. Based on the measured wall temperatures and emissivity values, the heat transfer from the walls for the operating conditions at the time of the measurements was estimated. The heat rejected through the walls amounts to approximately one fifth of the total heat loss. Secondly, when the furnace door is opened, a relatively large flow rate of hot gas leaves the furnace, and a net heat loss occurs due to the radiative heat exchange between the furnace interior and the environment. As the aforementioned heat losses are very difficult to measure, a simplified theoretical model was made based on physical principles. The corresponding results indicate that the opening of the furnace accounts for a large part of the remaining heat loss

Parametric study of a triangular cross corrugated plate

The main goal of this study is to investigate the influence of apex angle and Reynolds number on the thermal hydraulic performance of triangular cross corrugated plates. More specifically this work focuses on triangular cross corrugated plate with the orientation angle of 90°. The computational fluid dynamics (CFD) method is used to conduct, three-dimensional simulations for 426 < Re < 2021 in a periodic unitary cell. The Reynolds Stress model is used as the turbulence model. The numerical results are in a very good agreement with experimental results correlation. They show deviations between 0.8 – 4.84 %. The highest thermal performances are achieved by the both apex angle of 120° and 90°. The lowest thermal performance is observed by the apex angle of 55°. The heat exchanger with the apex angle of 90° has the highest friction factor. For the Reynolds number lower than 1300, the apex angle of 120° shows the lowest friction factor. However for the Reynolds number higher than 1300, the apex angle of 55° shows the highest hydraulic performance.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Flow regime based heat transfer correlation for R245fa in a 3 mm tube

241 heat transfer measurements for R254fa were conducted. The heat transfer coefficient was determined for a smooth stainless steel tube with an inner tube diameter of 3 mm. The experiments were conducted for five mass fluxes (100, 300, 500, 700 and 1000 kg/(m2 s)), three heat fluxes (10, 30 and 50 kW/m2) and at three saturation temperatures (40 °C, 70 °C and 125 °C). The experiments were used to determine the influence of the saturation temperature, mass flux, heat flux, vapour quality and flow regime on the heat transfer coefficient. At a low saturation temperature, the heat transfer coefficient increases with an increasing mass flux. However, at a high saturation temperature the heat transfer coefficient decreases with an increasing mass flux. Furthermore, the heat transfer coefficient increases with increasing vapour quality at a low saturation temperature. On the contrary, the heat transfer coefficient decreases at higher saturation temperatures. Due to the fact that most heat transfer models found in literature are developed for low saturation temperatures and one flow regime, the heat transfer coefficients predicted by the existing models do not comply very well with the experimental data. Thus, a new heat transfer correlation for R254fa was proposed. The new correlation has a Mean Absolute Error of 11.7% for the experimental data of a tube with an inner tube diameter of 3 mm. Finally, this new correlation was also verified with R245fa datasets of other authors

Potential of Organic Rankine Cycles (ORC) for waste heat recovery on an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology to convert low temperature waste heat to electricity. While several energy intensive industries could benefit from the integration of an ORC, their adoption rate is rather low. One important reason is that the prospective end-users find it difficult to recognize and realise the possible energy savings. In more recent years, the electric arc furnaces (EAF) are considered as a major candidate for waste heat recovery. Therefore, in this work, the integration of an ORC coupled to a 100 MWe EAF is investigated. The effect of working with averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC (SCORC) can generate an electricity output of 752 kWe with a steam buffer working at 25 bar. However, the use of a steam buffer also impacts the heat transfer to the ORC. A reduction up to 61.5% in net power output is possible due to the additional isothermal plateau of the steam

Steady state modeling of a fire tube boiler

Manufacturers increasingly strive to customize fire tube boiler designs to specific needs. A comprehensive thermal design model is therefore necessary. In this article a steady state thermal model based on the plug flow furnace model and the ε- NTU method is presented. The model includes the turn boxes which other authors neglect. The steady state model furthermore allows optimizing the boiler designs. It is used to analyze the gas temperature along the flow length. Secondly, the article compares a plug flow furnace model, the ε- NTU method with and without radiation. The ε- NTU with radiation allows decreasing the number of control volumes while retaining accuracy. Additionally the effect of the turn boxes is investigated.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers