"The helical strake thermowell referred to by Blake is one alternative for use in applications where a more traditional thermowell cannot be made to work. Velocity collars have been historically used as one means of changing the attributes of a thermowell to produce a suitable design for a given application. As mentioned in the original post, velocity collars have been recognized to be unreliable over the long term so are not recommended in the ASME PTC 19.3 TW – 2010 standard. There are other ways to change a thermowell design to ensure reliable service, and a helical strake thermowell design could be one solution to consider." - Jim Cahill