The effect of wall thermal conductivity on the critical flow velocity of the gas fired burner is illustrated in Figure 7 at a fixed external heat loss coefficient. The upper curve represents the high-velocity limit, resulting in blowout due to decreased convective timescales. The lower curve represents the low-velocity limit, resulting in flame stability loss due to reduced heat generation. Between these curves stabilized combustion is allowed, whereas outside the envelope, self-sustained combustion is impossible. Smaller wall thermal conductivities allow stabilized combustion for lower flow rates. Lower flow rates require less upstream heating and more insulation against exterior heat losses. At the other extreme, higher wall thermal conductivities result in maximum allowable flow rates, but the increased heat losses prohibit low flow rates. This relationship is important when designing devices. When a low-power device is desired, more insulating materials should be preferred. On the other hand, when a high-power device is desired, more conductive materials should be chosen. The present study focuses upon thermal stability. However, other material properties, such as allowable operating temperatures [59, 60], radical sticking [61, 62], and mechanical strength [63, 64], along with the burner efficiency should be considered when choosing a material for construction and designing burner dimensions [65, 66]. The inlet flow velocity plays a competing role in flame stability. Low flow velocities result in reduced power generation. On the other hand, high flow velocities decrease the convective timescale below that of the upstream heat transfer through the walls. As a result, there is only a relatively narrow envelope of flow rates within which combustion can be stabilized. When a low-power device is being designed, more insulating materials should be favored to minimize external heat losses. Conversely, a high-power device would favor more conductive materials.