mdotcold

mdotcold is a variable used in thermofluid modeling to denote the mass flow rate of the cold fluid in a two-fluid system. It is typically expressed in kilograms per second (kg/s) and is often defined as mdotcold = rho_c * Vdot_c, where rho_c is the density and Vdot_c is the volumetric flow rate of the cold stream. In many analyses, mdotcold is considered together with mdothot to describe the energy transfer between the streams through a heat exchanger or similar device.

In energy balance formulations, the heat transferred by the cold side is commonly written as Q = mdotcold * cp_cold * (T_in,cold - T_out,cold), acknowledging that cp_cold may vary with temperature. Under steady-state, constant cp assumptions, this equals the heat gained or lost by the hot stream, Q = mdothot * cp_hot * (T_out,hot - T_in,hot). In more general models, Cp can be a function of temperature, requiring integration along the temperature path.

In system design, mdotcold is determined by pumps and piping and is typically specified as an input or controlled variable. While the cold and hot streams exchange energy, their mass flow rates are not necessarily tied by mass transfer, but practical constraints, pressure drops, and control strategies can couple mdotcold and mdothot in a given circuit.

Measurement and estimation: mdotcold can be measured directly with mass-flow meters (such as Coriolis or thermal meters) or inferred from volumetric flow rate and density, with temperature corrections as needed. In compressible flows, mdotcold may vary with pressure and temperature along the circuit.

Applications: mdotcold is a standard concept in HVAC hydronic systems, refrigeration cycles, chemical processing, and solar thermal networks where a cold fluid circulates for heat exchange.