Research on improving the overall thermal performance of panel radiators based on the CFD method

A panel radiator is one of the radiators commonly used in central heating at present. Improving the heat performance of the panel radiator and making it more suitable for low-temperature heating is an important direction of current research on heating energy conservation. This paper studied the overall heat transfer performance of panel radiators based on the CFD method. The research shows that compared with the longitudinal continuous fin widely used at present, the use of discontinuous fins and staggered fins can improve the thermal properties of the panel radiator significantly. The comparative study of eight scenarios shows that the average heat transfer coefficient of the radiator equipped with longitudinal staggered fins and VGs increases by 120.24%, and the radiator equipped with discontinuous inclined staggered fins increases by 117.33%. Due to the reduction of heat dissipation area, the total amount of heat dissipation of the radiator equipped with discontinuous fins is not as good as the radiator equipped with staggered fins that does not change the heat dissipation area basically. This study provides a way to improve the heat transfer properties of panel radiators.


Introduction
The operating experience in recent years shows that reasonably reducing the heat medium temperature of the heating system in the building is conducive to improving the comfort level of building heating, reducing the thermal dissipation loss of the heating pipe network, achieving energy consumption reduction, promoting the application of new energy, and condensing boilers and other efficient equipment in the heating system.At present, renewable energy heating is developing rapidly, and the development of building heating equipment suitable for low-temperature heat sources is beneficial to promote the development and application of low-grade heat sources.Russia's new energy efficiency strategy (Russian Federation-Decree No. 1715-r) [1] defines the strategic goal of increasing sanitary hot water or cogeneration heating, so that the radiator water supply temperature in winter is 40℃, and the return water temperature is 20℃ to 22℃.Because China's centralized heating technology originated from the Soviet Union, for the radiator heating system, the recommended supply and return water temperatures in the new HVAC design standard [2] are 75℃ and 50℃ respectively.
From the radiator mechanism, the proportion of convection heat dissipation is greater than that of radiation heat dissipation, so the thermal response is faster.Changing the fin form of the plate heat exchanger to improve heat dissipation is one of the technical routes for heat transfer enhancement of the radiator.Calisir et al. [3] evaluated the influence factors of double plate double convection radiator on the heat output of PCCP from heat dissipation and weight to obtain the best design scheme.The various factors studied are shown in the figure below (Figure 1).On the whole, the total heat increased by more than 35% after installing the convectors on the panel radiator.When the fin thickness changes from 0.25 mm to 0.40 mm, the heat transfer rate increases by about 2.5%, and the material use increases by about 5% compared with the basic radiator.When changing the total height of radiator convection fins (H) from 400 mm to 560 mm, the heat transfer rate could increase by almost 4.5%, with an increase of 2.5% in weight.

Figure 1. Optimized parts of radiator fins
A classical way widely used in thermal engineering to improve thermal performance is to use staggered fins or channels.It is found that the air pressure loss in this way is relatively small, but the increased proportion of heat dissipation is relatively large [4,5].Adnan [6] found that the ventilated radiator could increase the air temperature from -5℃ to 26℃ when the water supply temperature was 45℃.Under the same conditions, the air outlet temperature of the radiator with continuous channels only increased to 21℃.Another method of heat transfer enhancement is to arrange vortex generators (VGs) near the heat transfer wall.In the field of vortex generator heat transfer enhancement, since 1969, Johnson [7] has performed extensive research to study the pressure drop and heat transfer properties enhancement caused by VGs.Several parameters, such as the chord length, angle of attack, Reynolds number, and aspect ratio have been considered.Tiggelbeck [8] conducted a comparative research of several types of VGs presented for 2000 <Re<9000.Chen et al. [9] conducted an experimental study on the flow and heat transfer performance of the CFD-type longitudinal vortex generator installed in the microchannel.Ebrahimi et al. [10] conducted a numerical simulation study on the flow field and heat transfer properties of the micro-channel with CFD-type longitudinal VGs.The results show that the heat transfer performance is 2.0%~25.0%higher than that of the optical channel, and the resistance performance is 4.0%~30.0%higher.Tang et al. [11] calculated and compared the flow and heat transfer performance of the finned tube fin heat exchanger of the CFD and CFU longitudinal VGs, and found that the Nu of the CFU longitudinal vortex generators increased by 2.8% on average, while the f factor decreased by 9.1% on average, compared with the CFD longitudinal vortex generators.
In terms of research on performance improvement of heating equipment, there are rich research achievements.The improvement mainly focuses on the following aspects: 1) The optimization of the shape and size of heat dissipation elements of heating equipment mainly refers to the optimization and improvement of the shape, size, and layout of heat dissipation fins.2) In terms of the research on the heat performance improvement of the convection radiator, many countries are keen on the research on the forced convection radiator, and there are many achievements in the improvement of the thermal performance of the ventilation radiator.In this regard, there is a lack of relevant research in China.China's achievements mainly focus on the technical means of strengthening natural convection.It includes changing the overall structure size and shape of the radiator, adopting wall treatment (vortex generator), etc. to improve the heat dissipation capacity of the radiator.These rich basic studies have laid the foundation and pointed out the direction for this study.

Methodology
In this paper, the improvement of heat dissipation capacity after the optimization of fin form and the installation of a vortex generator is studied.Therefore, an improved technical route for the thermal properties enhancement of the radiator is given.The CFD method is used to study the thermal performance improvement path of the radiator, and the calculation software is STAR-CCM+ of Siemens.Simcenter STAR-CCM+ can simulate fluid flow for a variety of fluid types.The testing platform used in this article is built according to the EN442-2 standard.(Figure 2, Figure 3).The surrounding walls are water-cooled walls.Heating balance with the radiator is achieved by controlling the wall surface temperature.The inner size of the test room is 4 m×4 m×2.8 m.The measurement accuracy of air temperature is ±1.5%.The thermal resistance temperature sensor (PT100) is used to test the supply and return water temperatures of the radiator.The measuring range is -100℃~200℃, and the accuracy is ±0.4%.The weighing method is used to obtain the water flow rate.
In this paper, the CFD method is used to study the thermal performance improvement path of the radiator, and the calculation software is STAR-CCM+ of Siemens.Simcenter STAR-CCM+ can simulate internal and external fluid flow for a variety of fluid types.
The continuity equation is used to describe the mass balance of the computational domain: where ρ is the mass density, and v indicates the speed of the calculated object.The momentum balance equation is defined as follows: where  denotes the outer product, σ is the stress tensor, and fb is the resultant of the body forces per unit volume.For fluids, stress tensors can usually be decomposed into normal stress and shear stress: where T is the viscous stress tensor and p is the pressure: The stress tensor should satisfy the symmetry condition: The energy equation of the control body can be written as: where q is the heat flux, E is the total energy per unit mass, and SE is an energy source per unit volume.
The Realizable Two-Layer K-Epsilon model [12] was used to calculate the turbulent characteristics of the flow field, We can enhance the convergence of natural convection simulations by using the Boussinesq model.If the constant density ref  is used in all terms of the governing equations by eliminating  through the Boussinesq approximation the buoyancy source term is approximated as: where Tref is the operating temperature, and ref  is the reference density.
The local heat transfer coefficient is calculated as shown below: where Tw is the heat exchange wall temperature, Tf is the fluid temperature, and qw is the heat transfer through the wall.The average heat transfer coefficient of the panel surface is calculated as follows: In this paper, we combined the segregated energy model and the Surface-to-Surface radiation model together to determine the temperature field.The radiation heat transfer between two surfaces depends on their size, distance, and direction.These parameters in the software are represented by geometric functions of "view factors".The emissivity of room wall surface and radiator surface was set to 0.9 and 0.88 respectively.We use the following conditions to judge that the calculation reaches a steady state: the total heat dissipation amount of water shall be equal to the heat dissipation amount of the radiator to the air domain.

Case Setup
In this paper, the room was simulated with the size of 4 m×2 m×2.8 m respectively according to the EN 442-2 standard.The boundary condition settings of the CFD model are shown in Figure 4.The width is half that required by EN442 standard.The panel radiator used in the laboratory test is 500 mm long, 60 mm wide, and 600 mm high, with 15 water passages distributed inside.The installation style of the radiator in the laboratory is shown in Figure 5.The heat insulation boundary condition is set for the wall where the radiator is installed, and the temperature boundary condition is set for the top surface, the ground, and the remaining three surrounding walls, with a temperature of 18℃.The water inlet temperature of the radiator is 80℃, the flow is 7 g/s, and the water outlet is the pressure outlet.The room temperature of 18°C was obtained with the surface temperature of five walls set at 18℃.The fan is arranged at the lower part of the radiator, the outlet of the fan is a velocity boundary condition with a velocity of 1m/s, the inlet of the fan is a pressure boundary condition, and the surrounding walls of the fan are adiabatic boundary conditions.In this paper, the present experimental results were used to validate the CFD results (Table 1).Four prototype models were verified.The inlet/outlet temperature and heat output values were verified in the table.The maximum thermal output deviation of prototype 5 was 0.88% at 55/45℃.In addition, under the condition of 80/60℃, the front panel temperature distribution of the reference radiator was obtained by infrared thermal imager as shown in Figure 6.The comparison of experimental values and simulation results is shown in Figure 6.The comparison pictures show that the temperature distribution of the two is consistent.The value difference between the measured results obtained by the thermal imager and the CFD simulation results at the same location is small.It could be concluded that all thermal output values obtained by numerical calculation were within the range of experimental uncertainty.CFD simulation results are reliable.The thermal performance of radiators with different supply and return water temperatures under natural convection conditions was studied by laboratory test methods, and compared with the CFD simulation results.These experimental results were used to verify the numerical simulation results, which were in good agreement.
4 Study on the Improvement of the Overall Heat Dissipation Performance of Panel Type Radiators On the basis of the previous research, this paper selects a staggered fin panel type radiator for overall thermal performance research and studies the thermal performance of this type of radiator with a vortex generator and discontinuous fins.In several cases, the fin form of the radiator is as follows.The longitudinal staggered fin is cut the longitudinal continuous fin into four parts, and the adjacent two parts are offset by 6 mm; Discontinuous longitudinal staggered fins remove 30 mm wide fins at the truncation point of longitudinal staggered fins.The longitudinal continuous fins are truncated into four parts, and the fins are inclined and offset by 25° to form inclined staggered fins.The discontinuous distance of discontinuous inclined staggered fins is also 30 mm (Figure 7-Figure 9).Based on these three types of fin forms, this paper considers whether the fin is continuous in the middle and whether an eddy current generator is added to form eight types of plate radiators with different fin forms.The thermal performance of the radiator is simulated with STAR-CCM+ software.It can be seen that the heat dissipation is improved by using staggered fins to reform the longitudinal fins of the original radiator (Table 2).The heat dissipation of the radiator with longitudinal staggered fins increased by 6.6%, the average heat transfer coefficient of the radiator in contact with the air increased by 15.48%, the heat dissipation of the radiator with inclined staggered fins increased by 5.82%, and the average heat transfer coefficient increased by 12.24%.After arranging vortex generators on these two types of finned radiators, the heat dissipation increased to 8.83% and 8.33% respectively, and the average heat transfer coefficient increased to 20.24% and 17.71% respectively.
When discontinuous fins are used, for the longitudinal staggered fin radiator and the inclined staggered fin radiator with a gap width of 30 mm, the heat dissipation increases by 1.07% and 2.24% respectively, relative to the reference radiator (Table 3); The average heat transfer coefficient increased by 16.63% and 17.33% respectively; however, the fin area of these two types of radiators is reduced by 18%.This shows that the use of discontinuous fins can significantly improve the surface heat transfer coefficient of the radiator, to a certain extent, make up for the reduction of heat dissipation due to the reduction of heat dissipation area.In this example, the fin area is reduced, but the overall heat dissipation is not reduced.This shows that the optimization of radiator fins can effectively reduce the steel consumption of radiator manufacturing, thus creating benefits for radiator enterprises.
When a vortex generator is used in a finned staggered radiator, it has little effect on the overall heat dissipation performance (Figure 10).This is because the interlaced fins of the staggered radiator have the function of a vortex generator, which has enhanced the disturbance in the original flow field and improved the heat transfer boundary conditions.Comparatively speaking, for the radiator with longitudinal continuous fins, if the vortex generator is directly added, the heat dissipation will increase by 5.19%, and the average heat transfer coefficient will increase by 8.63% (Table 3).The effect is significant.Therefore, in practical application, it is not recommended to use a vortex generator and staggered fin technology simultaneously to improve the thermal performance of the radiator.Considering the improvement effect and the convenience of technology application, staggered fins can be preferred to optimize the heat performance of the panel radiator.

Conclusions
In this paper, the CFD method is used to research the function of the vortex generator in improving the heat transfer coefficient of the panel radiator.The study is focused on forced convection and the radiator inlet speed is set as 1 m/s.Based on the CFD model established by EN442 standard laboratory, this paper studies the fin optimization of the TYPE11 panel radiator.Without changing the fin heat dissipation area, the heat dissipation of the radiator with longitudinal staggered fins increased by 6.6%, and the heat dissipation of the radiator with inclined staggered fins increased by 5.82%.When discontinuous fins are used, the average heat transfer coefficient increased by 16.63% and 17.33% respectively for the longitudinally staggered fin radiator and the inclined staggered fin radiator.
The comparative study of eight scenarios shows that the average heat transfer coefficient of the radiator equipped with longitudinal staggered fins and VGs increases by 120.24%, and the radiator equipped with discontinuous inclined staggered fins increases by 117.33%.Due to the reduction of heat dissipation area, the total amount of heat dissipation of the radiator equipped with discontinuous fins is not as good as the radiator equipped with staggered fins that does not change the heat dissipation area basically.This study provides a way to improve the heat performance of panel radiators.
Considering the interlaced fins of the staggered radiator have the function of a vortex generator, which has enhanced the disturbance in the original flow field and improved the heat transfer boundary conditions, it is not recommended to use vortex generator and staggered fin technology simultaneously.

Funding:
This research was financially supported by the National Key Technology R&D program in the 13th Five Year Plan of Study on Solar Heating and Heat Storage Technology in Rural Area (2018YFD1100701).

Figure 2 .
Figure 2. The structure of test bench.Figure3.Appearance of test bench.

Figure 3 .
Figure 2. The structure of test bench.Figure3.Appearance of test bench.

Figure 4 .Figure 5 .
Figure 4. Thermal performance analysis model and boundary conditions of radiators

Figure 10 .
Figure 10.HTC of staggered fins radiators with VGS

Table 1 .
Results comparison for different Prototypes Figure 6.Temperature distribution of experimental and CFD results

Table 3 .
Improvement of thermal performance of radiators with different fin structures