control thermal zone temperture by air flow rather than chilled water flow

edit

District chilled water tariff is normally charged not based on the flow rate but based on the energy used. The cooling energy that the district chiled water provides is modulated to meet the cooling load. I don't think the chilled water flow rate matters, but if you don't want to reduce the flow rate, you can use a constant-speed pump for DCS rather than a variable-speed pump. The district chilled water flow rate will be constant at the max flow, and the delta T of the district chilled water will decrease. The on-site chilled water pump should be modelled as per design.

If you want to reduce the fan flow rate, I recommend reviewing your supply air temperature control. Note that Warmest i.e. SAT reset control increases the fan flow rate. The intention of SAT reset control is 1) to avoid overcooling and/or 2) to allow high chilled water temperature which results in higher chiller COP, but now your model uses DCS with a constant supply water temperature. Everything should be considered comprehensively for energy optimisation, but if you just want to minimise the fan flow rate, the supply air temperature setpoint should be fixed.