Colloquia

Samenvatting

In view of the variations found in renewable domestic hot water heating sources, supplementing gas fired low and high heating output devices are used to satisfy demand. Integrating both capacities results in a single device with a larger operating range. Additionally, the larger operating range results in more efficient and durable devices. Therefore, in this research the goal was to increase the operating range of gas fired hot water appliances.

To achieve such larger operating range, the gas mixer is taken as a research subject. Here, air and gas flows are coupled pneumatically. The coupling is a result of the venturi effect which creates a so-called signal pressure on the gas valve. Based on this effect, a lower gas flow rate can be achieved by decreasing the overall flow rate, and a higher flow rate by increasing the overall flow rate. Both these can be realized by changing the pressure drop over the gas mixer while maintaining enough signal pressure. For this purpose, multiple concepts were developed. The research concept was selected to be a translating gas nozzle in the venturi. By translation, the internal geometry alters and the cross sectional area of the venturi is changed by 15%. With these changes, the pressure drop and signal pressure are expected to change.

To investigate the gas mixer behavior more in detail, a pneumatic test was developed. In the pneumatic test, the pressure drop and signal pressure were measured and compared for different gas nozzle positions and volumetric flow rates. From this, the changing cross section was found to have almost no effect on the full system pressure drop at all flow rates. For the gas mixer itself however large changes (33 - 44%) were found. The influence on the signal pressure was also large (66 - 50%). 

To verify these results, an operational test was conducted. Here, the increase in cross sectional area in the gas mixer results in a heating output increase at constant gas to air ratios of 5% at low loads, and 3% at high loads. From this it can be concluded that improvements in the operating range can be made by decreasing the cross section of the gas mixer at low loads and increasing it at high loads. Furthermore, changes in the gas to air ratio can be achieved by changing the internal geometry. 

In conclusion, the research clearly shows a proof of concept for the translating nozzle concept. At this moment, the improvements are not large enough yet for actual implementation in products. However, a basis is laid for performance evaluation of gas mixers based on pneumatics.