The effect of the ceramic foam cell structure on the combustion performance of porous media burner: experimental study

To investigate the effect of the ceramic foam cell structure on the combustion characteristics, a porous media burner(PMB) composed of foamed ceramic plates with Kelvin and WP cell models as structural features was designed and developed. The combustion chamber temperature, lean combustion limit, radiation efficiency and flue gas emission characteristics of the burner were analysed in the combustion of methane. The results show that with the same pore density, skeleton diameter and operating parameters, the combustion chamber temperature and radiation efficiency of the WP model are higher than those of the Kelvin model, while the CO and NOx emissions of WP model have an opposite tendency. When the ceramic foams are composed by WP cell model, under the same pore density and operating parameters, the burner has better combustion performance when the skeleton is thin. The combustion chamber temperature and radiation efficiency are higher, CO and NOx emissions are lower, and the lean combustion limit equivalent ratio is lower.


Introduction
Among many combustion technologies, porous media combustion(PMC) has outstanding superiority with high combustion efficiency, low pollutant emission and wide range of combustion limit, etc.At present, PMBs are widely used in various fields [1,2].
The porosity, pore diameter and pore density of porous media have an important influence on the PMC and also on the performance of the relevant porous media burners.many researchers have carried out targeted studies.Wang et al. [3] used alumina particles as porous media and found that the temperature in the reaction zone decreased with increasing porosity under the same conditions.Ding et al. [4] studied the effect of pore density of porous media on combustion, and the results showed that the complete combustion degree was best and the heat transfer effect was better when the pore density was 20 PPI.
Essentially, the structural parameters of porous media depend on the morphology and distribution of the pores and the solid skeleton of the porous media at the microscopic level.Therefore, several researchers have studied the effect of the microstructure of porous media on combustion.The effect of foam ceramic material and solid skeleton size on the combustion characteristics of porous media was modelled by Tan et al. [5] using numerical methods.The structure of the foam ceramics was described by a decahedral array and the results showed that the temperature was highest when the material was SiC and that the nodal edge length and the radius and length of the skeleton pore rib had an important effect on the combustion temperature distribution.The structural design was found to be a good alternative to disordered foams with ordered porous structures that can better control porosity and structural stiffness.Decahedral diamond lattice structures have better combustion performance [6,7], and the geometry is homogeneous and does not generate combustion instabilities such as partial combustion.
A review of the literature indicates that the combustion characteristics of PMC, based on the microstructure of the porous media, have not been adequately investigated and experimental studies are scarce.In this paper, the Kelvin model and the WP (Weaire-Phelan) model, which are recognized by scholars as capable of describing the cellular structure of foam-type porous media, were used to construct a 3D model of a foam ceramic plate using the Siemens NX 3D software.The alumina foam ceramic plates were fabricated using 3D printing technology.A PMC experimental platform was built, and the effect of the cellular structure of the foam ceramic plate on the combustion performance of PMC was investigated.The results of the experimental study can provide both experimental validation for related numerical simulations and it can support and theoretical guidelines for designing the ceramic foam used in PMCs.

Experimental system
The experimental system, consisting of a gas supply system, a PMB and a data acquisition system, is schematically shown in Figure 1.The gas supply system consists of fuel (methane gas, 99.99% pure) and air (79:21 ratio of nitrogen to oxygen flow).A pressure reducing valve controls the outlet pressure of all three gases equally, and a computer-controlled flow meter (SevenStar CS200) provides precise control of the flow to within ±1.0%.Nitrogen and oxygen are mixed uniformly in premix chamber II to form air, and air and methane are mixed at a set ratio in premix chamber I then enter the combustion chamber for combustion.The PMB is shown in Figure 2, the burner shape is composed of a square tube of 30mm x 30mm length, the wall thickness of the square tube is 2mm, the hight is 100mm.The outside of the square tube is wrapped with high temperature resistant cotton fibre to reduce heat loss.The porous media burner uses straight-hole foam ceramic plates with a spray hole diameter of 0.8 mm for uniform air distribution upstream and 3D printed square foam ceramic plates with a side length of 1 inch and a thickness of 4.6 mm downstream.
Type K thermocouple is installed in the centre of the PMB at a distance of 10mm from the foam ceramic plate to measure the integrated temperature within the chamber.The thermocouple is connected to a data collector and enables real-time monitoring of the internal temperature of the burner via a PC.The thermocouple has a measurement range of 0-1300°C and an accuracy rating of ±1.5%.The thermal imaging camera is a Fluke Tix650 with a temperature measurement range of -40°C to 1200°C.The thermal image can be used to determine the temperature distribution on the surface of the foam ceramic plate and to calculate the average surface temperature.
Figure 3 shows the thermal image at φ=0.9 and q=300ml/min.The average surface temperature analysis in the following section is based on the thermal image data.A Testo 350 flue gas analyser is used for flue gas composition analysis and temperature measurement.

Experimental procedure
During the experiment, after adjusting the methane and air flow rates, open the three gas cylinder valves and simultaneously switch on the temperature measurement system.Waiting for the gas to completely fill the combustion chamber and then ignite.To avoid the influence of the ignition position on the combustion, keep the same ignition position each time and ignite from the top of the stainless steel outer housing, the flame will spread from upstream to burn near the surface of the porous medium.The thermocouple records the integrated temperature change in the combustion chamber, if the thermocouple temperature change rate is less than 1°C/min, the combustion is considered stable, the flue gas composition and flue gas temperature are measured with a flue gas analyser and an infrared thermal image is taken with an infrared thermal camera.To measure the equivalent ratio of the lean combustion limit, the burner is first allowed to burn steadily at an equivalent ratio of 1 under certain flow conditions, and then the flow meter is adjusted to change the magnitude of the equivalent ratio until the flame is extinguished and the lean combustion limit is measured.The radiation efficiency is calculated based on the formula provided in [8].The surface emissivity of the foam ceramic plate is 0. 33 [9].
For the cell structure, two cell structures were used as shown in Figure 4, the Kelvin model on the left and the WP model on the right [10,11].
The alumina foam ceramic plates shown in Figure 5 was produced using 3D printing technology with the specific parameters shown in Table 1.Considering that PMBs are mostly lean combustion limit in their applications, the working conditions for this experiment are shown in Table 2.

Combustion chamber temperature
Figure 6 shows the variation of combustion chamber temperature and flue gas temperature with equivalence ratio under a flow rate of 300 ml/min for #1~#4 different PMBs.As can be seen, both the combustion chamber temperature and the exit flue gas temperature increase with increasing equivalence ratio, but there is little difference in the stable combustion chamber temperature for equivalence ratios of 0.9 and 1.At the same equivalence ratio and methane flow rate, the highest stable combustion chamber temperature is found in #2, followed by #3 and #1, and the lowest in #4.
The highest exit flue gas temperature is found in #4, followed by #1 and #3, and the lowest in #2.The temperature measured by the thermocouple is a combined average of the entire combustion chamber temperature, which is influenced by the radiation from the porous media plates and walls, high temperature flue gas convection and radiation [12].Under certain conditions of methane flow, the temperature tends to increase as the equivalent ratio increases and the flue gas heat loss decreases.However, at an equivalence ratio of 1, the degree of methane ignition is slightly less than the ignition rate at an equivalence ratio of 0.9, so although its exhaust loss decreases, the temperature does not differ much from the working condition at an equivalence ratio of 1.
Figure 7 shows the variation of combustion chamber temperature and flue gas temperature with different methane flow rate for an equivalent ratio of 0.9.The combustion chamber temperature and flue gas temperature increase with increasing methane flow rate for all four operating conditions, with the highest combustion chamber temperature for #2, followed by #3 and #1, and the lowest for #4.Flue gas temperature, on the other hand, is lowest in #2, followed by #1 and #3, and highest in #4.
As can be seen from Figures 6 and 7, when the skeleton diameter is both 0.4 mm, the combustion chamber temperature is higher for the WP model ( #2) than for the Kelvin model ( #1) for the same equivalence ratio and the same methane flow; when the plate cell structure is both WP models, the combustion chamber temperature is higher for the same equivalence ratio and the same methane flow rate with a thinner skeleton diameter, while the exhaust smoke temperature is lower.With the same skeleton diameter, the WP model has a larger plate porosity, which results in a lower resistance to flow and a higher effective thermal conductivity [13], as well as a lower radiative attenuation coefficient, which results in more heat being returned in the form of conduction and radiation, and a higher premixed gas preheat temperature, resulting in a higher burner temperature and lower exhaust smoke temperature.Similarly, when the plate cell structure is all WP model, as the skeleton diameter increases, the porosity gradually decreases, the radiation attenuation coefficient increases, which is not conducive to the radiation of heat downstream in the combustion region [14], the average temperature of the porous medium decreases, the preheating effect of the premixed gas becomes worse, the flame temperature decreases, the combustion chamber temperature is lower, and the flue gas heat loss increases.

Radiation efficiency
Radiation heating is the primary heating method of PMBs and radiation efficiency is an important measure of combustion performance.The radiation efficiency of the PMB can be calculated using radiation efficiency calculation formula, where the temperature of the ceramic foam plate can be statistically derived from the temperature values in the infrared thermogram.At a gas flow rate of 250 ml/min, the radiation efficiency of the four burners varies with the equivalent ratio as shown in Figure 8.As can be seen, under the same flow rate and equivalent ratio conditions, the radiation efficiency of the PMB #2 is the highest, #3 is the second highest, and #1 and #4 are lower.At the same time, as the equivalent ratio increases, the radiation efficiency of different burners shows a monotonically increasing trend.Under the same conditions of methane flow rate, the radiation efficiency of the PMB is related to the average temperature of the ceramic foam plate, the higher the temperature, the higher the radiation efficiency.From the experimental results, it can be seen that for the #2, when the gas flow rate is 250ml/min, its radiation efficiency is about 16%~18%.
The variation of the radiation efficiency of the four PMBs with the methane flow rate when the volume ratio is 1 is shown in Figure 9.It can be seen that as the methane flow rate increases, the radiation efficiency of the burners all show a decreasing trend.It is known from the previous analysis in Figure 7 that as the methane flow rate increases, the average temperature of the combustion chamber increases and the numerator in radiation efficiency calculation formula will increase, but the increase in methane flow rate also causes the denominator in radiation efficiency calculation formula , which represents the supply load, to increase, indicating that the rate of increase in radiation efficiency due to the increase in plate temperature is not sufficient to compensate for the increase in fuel supply load and the combined effect is a decrease in radiation efficiency.

lean combustion limit
In this experiment, after achieving stable combustion at an equivalent ratio of 1, the equivalent ratio was adjusted to decrease in steps of 0.1 until the premixed flame was extinguished and the lean combustion limit equivalent ratio was measured.PMC has good performance in handling the combustion of some rarefied gases, and testing the lean combustion limit is important in assessing the performance of porous media.
Figure 10.Variation of lean burn limit of porous medium burner with methane flow Figure 10 shows the experimentally measured variation of the equivalent ratio minima with combustion intensity.As can be seen, the minimum equivalent ratio decreases with increasing methane flow rate, indicating that the higher the flow rate, the lower the minimum equivalent ratio required to maintain stable combustion of the flame and the wider its lean burn limit.For different PMBs, #2 has the widest lean burn limit for the same operating conditions, with #3, #1 and #4 becoming narrower in that order.In this experiment, the measured minimum equivalent ratios for lean combustion at different methane flow rates range from 0.29 to 0.5, all of which are lower than the lean combustion limit equivalent ratio of the free flame (which is 0.53 [15]), indicating that PMC has a great advantage in rarefied gas combustion.

Emissions of pollutants
The pollutants emissions of PMBs are an important parameter in assessing their performance.This experiment compares the differences in pollutant emissions for four different configurations of PMBs. Figure 11 shows the variation of CO emissions with equivalent ratio for the four burners at a gas flow rate of 300 ml/min.As can be seen, for the same skeleton diameter, the PMB #2 emits less CO, and for the same model, the #2 PMB emits the least CO, #3 the least, and #4 the most.As the equivalent ratio increases, the CO emissions first decrease and then increase and are lowest at an equivalent ratio of 0.9.It can be seen that under lean-burn conditions CO emissions are positively correlated with the combustion chamber temperature and that CO emissions can be predicted from the combustion chamber temperature.At an equivalent ratio of 1, CO oxidation is incomplete, resulting in higher CO emission concentrations.Figure 12 shows the variation of CO emissions with gas flow rate for the four different burners at an equivalent ratio of 0.9.It can be seen that the CO emissions of the four burners decrease with increasing flow rate and reach a minimum at a methane flow rate of 300 ml/min.The Kelvin model plate (#4) has the most significant variation in CO emissions with gas flow fluctuations, while the WP model thin skeleton plate (#2) has no CO emissions greater than 60 ppm and its variation with gas flow fluctuations is the smoothest, indicating that the WP model plate is more suitable for engineering applications.
Figure 13 shows the NOx emission levels for different operating conditions.There are three main mechanisms for NOx generation, namely thermal NOx, fast and fuel based.In this experiment, thermal NOx is the main generation mode and thermal NOx emissions are temperature dependent, with emissions increasing with temperature.From the experiment it can be seen that the NOx emissions from #2 are slightly higher than the others, but their emissions are all at a low level, with the highest NOx emissions being only 3 ppm, which fully demonstrates the advantage of the PMB in terms of low pollutant emissions.

Conclusion
In this paper, combustion experiments of PMBs with different structures have been carried out and the effect of skeleton diameter and model of porous media cell element structure on the performance of PMBs has been studied.The main conclusions are: (1) Under the same combustion conditions and skeleton diameters, the combustion chamber temperature and radiation efficiency of the WP model combustion chamber composed of foamed ceramic plates are higher than those of the Kelvin model, and the depleted combustion limit of the WP model is wider and the CO emission is lower than that of the Kelvin model.
(2) Under the same combustion conditions and models, the WP model had better combustion performance with a thinner skeleton, higher combustion chamber temperature and radiation efficiency, wider lean combustion limits and lower CO emissions.
(3) The combustion chamber temperatures of the four different burners increase with increasing equivalent ratio, with temperatures close to each other at equivalent ratios of 0.9 and 1. CO emissions decrease and then increase with increasing equivalent ratio, being lowest at 0.9.The lean burn limit decreases with increasing combustion intensity.

Figure 6 .Figure 7 .
Figure 6.Change of combustion chamber temperature and flue gas temperature with equivalent ratio

Figure 8 .Figure 9 .
Figure 8. Variation of radiation efficiency of four PMBs with equivalent ratio

Figure 11 .Figure 12 .
Figure 11.Change of CO emission of porous medium burner with equivalent ratio

Figure 13 .
Figure 13.NOx emission level of four PMBs

Table 1 .
Parameters of foam ceramic plates