Yes, it is possible. This is because in the calculation of heat flux at flow boundaries, heat of formation is also included. The energy flux out of a boundary is calculated as the integral of the mass flow rate times the total enthalpy. The total enthalpy contains energy from thermal, chemical, radiation and kinetic energy. This can easily be a negative quantity. Ignoring radiation and kinetic energy, this flux is: integral ( m_dot * h ) dA m_dot is mass flow rate, A is face area and h is the enthalpy, defined as: h = Sum ( Y_k * h_k ) where Y_k is the k’th species mass fraction, and h_k is the k’th species enthalpy defined as: h_k = h_k^o + integral_T0^T ( cp_k dT ) where h_k^o is the enthalpy of formation, T0 is the reference temp, and cp_k is the specific heat of species k. This definition of enthalpy is completely relative and it is possible to get negative values. For example, an air stream with O2 and N2. The enthalpy of formation is 0 for both O2 and N2, and hence if the inlet temperature T is less than the reference temperature T0, the enthalpy will be negative (typically T0 is 300K).