fig6 such features can be internal to the passages such a raised chevrons turning vanes trip strips swirl features guide vanes or other flow disruptors or can be external to the passage such as screens or meshes immediately adjacent to or offset from the face of the honeycomb array but normal to the array axis to create turbulence in the flow field that enters the honeycomb array by another aspect of the invention the plurality of substrates can be placed immediately adjacent to one another or offset by a gap which may contain further flow disruptors the disruptors 16 extend from the wall of the honeycomb and into the flow field to create karman instability through vortex shedding the shedding of vortices is in effect a turbulence generator which induces mixing and leads to the desired improved contaminant mass transport to avoid a shadowing of the photocatalyst the protuberances are preferably made of uv transparent material their location may be at the entrance of the honeycomb or at an intermediate location on the cell walls as another means of introducing protuberances at the honeycomb entrance an interlaced grid 17 or mat like construction e g screen can be positioned against the entrance face of the honeycomb as shown in fig7 a and 7b the screen 17 could be offset by a small distance sufficient to create and maintain turbulent flow fields downstream of the screen 17 as shown in fig7 c or be located immediately adjacent the entrance side to the honeycomb array as shown in.