The phrase "flared up" describes a sudden and intense burst of emotion, energy, or activity. It is commonly used to describe situations that become volatile or contentious. Synonyms for "flared up" include erupted, exploded, ignited, kindled, sparked, and blazed. These words suggest a violent and sudden outburst of emotion or activity that may occur without warning. Other possible synonyms for "flared up" include intensified, heightened, surged, escalated, and spiked, which suggest a more gradual or steady increase in energy or intensity. Overall, the various synonyms for "flared up" can help to convey a range of different tones and nuances, depending on the context and intended message.