Sensitive y+ grid adaption

Marc

Dear CFD Users,
I am using fluent to calculate the heat loss of a long tube in a cold surrounding, turbulence model is SKE. The fluid is air, because of low pressure loss I am using an incompressible ideal gas law. The temperatur of the air will decrease from 500°C to 200°C, therefore the fluid properties change significant. I generated the wall cells that the values of y+ lie in the range of 20 < y+ < 60.
The problem:
The hex cell height is constant. The value of y+ changes along the tube - not much, but enought to get inaccurate results. When I use the automatic adaption in fluent, some cells get divided others not. This leads to unsteady (when comaring them along the tube) wall cell heights, the overall behaviour is worse. Is there a possibility to adapt the wall cells in a proportional way, to achieve a smaller range y+ (28 < y+ < 30)? Therefore it would be neccessary to stretch or compress the wall cells by, for instance, 10 % . Any hint?

Thanks in advance,

Marc